Daric:
1) The Rules -- The basic differences in the actual rules of the games can be thought of as going from cinematic to realistic. 40K is the most cinematic game I know of, Warzone a little more realistic but still "colorful", LoS a still slightly more realistic and way on the other end of the spectrum comes StarGrunt II.

Allan:
Agreed. 40K is actually quite silly in some of it's assumptions, such as the range at which weapons can fire. WH40K is space fantasy, heavy on the fantasy. Warzone is similarly space fantasy. Stargrunt II is hard SF, to the point where some of us have been working on using it for historical games.

Daric:
In 40K you have powerful characters that are the center pieces of your army, loaded with exotic weapons. On the other side, SG II does not have rules for individual characters (except snipers). There are leaders but their fighting value is the same as a line trooper. Warzone does have Big Ragu type characters, but if a whole squad of grunts opens up on him, he will die. All three games ( I have not played LOS much at all) are good at what they do, as far as that goes it's personal preference. I consider the flavour of Warzone and 40K to be close and I prefer Warzone rules. But when I want the feeling of a few scared young men, crawling on their bellies in the mud and wishing they were somewhere else, you can't beat SGII.

Allan:
In WH40K your leaders are usually your most powerful characters. This causes great laughter amongst those in the armed forces. The leaders should be LEADING. Basically, it's Dark Ages combat thrown into the far future. The leaders "lead" by charging the enemy and bashing more heads than anyone else. This kind of combat died with the Vikings, but it's been resurrected here. This can be lots of fun, if it's your cup of tea. Stargrunt II (henceforth SG2) is a lot heavier into things like morale and leadership. SG2 also includes neat morale effects if you happen to leave your wounded buddy lying in the mud screaming without getting him medical assistance...

The rules of all three are relatively easy to pick up, but WH40K seems to be the most fiddly. GW keeps adding new equipment and new rules that tend to throw the game's balance a little out of whack, so there are a lot of things to look up in different places. Note that Games Workshop has a tendancy to "re-write" its games every couple of years. If you want to play in their stores, you have to use the current figures with the current rules. I have the old Rogue Trader version (which plays quite a bit differently from the latest version) but no one plays it anymore.

Warzone's core rules are pretty straightforward, with two additional supplements for things such as vehicle rules. Both of these games have a point system.

Daric:
When it comes to playing the games, I find most of the battle is won or lost in the army selection phase for 40K. Aside from some very lucky or unlucky die rolls. But most of the time, someone familiar with the game can tell who should win after both sides are set up. Strategy is very simple when you get your army down. In Warzone it's about equal between troop selection and game play. You can come up with some killer combo of troops and equipment, but if they walk in front of my basic grunts they are toast. In Stargrunt II it is recommended that you come up with scenarios in which you (like a real battlefield commander) don't have whatever troop type you like. They don't have a point system or anything. SG II is not about competetion but the play of the game. Noble, but I'm not quite there yet.

Allan:
This is true, though some people are cobbling one together and I wouldn't be surprised to see one show up in the first supplement. The game is based on scenarios, similar to what the historical guys do. It's harder to get into, but results in more interesting games.

Note, though, that SG2 only has the one book. That's all you need. It's a generic game designed for use with any figures. It comes complete with vehicle rules, air support rules, artillery rules, etc. It doesn't have rules for aliens (they will be in the first supplement).

Daric:
2) The Figures -- GW has the highest quality and price. I think the average is around $3.50 per basic fig. More for Sgts and specials. With single characters going up to $9 Vehciles and such range from $12.50 (plastic bike) to upwards of "Hell I can't count that high!" And they seem to be very effective at brainwashing you into appreciating ludicrously oversized weapons, excess ornementation, and Arnold type muscles. Hell, I fell for it! Warzone does produce some very nice minis but you sometimes have to pick and choose. Difficult because few stores carry their entire line (quite large at this point!) They also have oversized weapons (but at least recognizable as based on modern tech) and huge muscles but ornementation is lower. Thus the figs are easier to paint up to a high standard. Prices are lower for Warzone figs (slightly over $2 per fig) Most are packed as $9 for four. Leaders are not too much higher.

Allan:
Not surprisingly, to play WH40K in GW sanctioned tournaments and the stores, you need the latest figures. A number of people outside of the stores require this as well, but your game group may be different.

Daric:
SG II has some nice figures, so I've been told. Official figs are highly optional (It says so in the rules!) The few figs I've seen, I did not like. But that might just be because they are true 25mm, have reasonable sized guns and everybody does not have "huge pectoral muscles". Even cheaper than Warzone because they are much smaller.

Allan:
But you can also use any figures you can find for SG2. I'm about to start painting some old 15mm Traveller figures for SG2. I also have some not-as-old 25mm Leading Edge Games figures from their Living Steel RPG. You can easily scrape together any figures you need or want. There are a number of conversions out there for using WH40K figures with SG2, for those of us that cut our teeth with GW but have become disillusioned with the GW money machine.

Daric:
3) Finding opponents - You must consider this as a great game that sits on your shelf is not such a great game after all! (Anybody for a game of Flintloque???) GW wins hands down right now. You go to any place that has in-store gaming and you'll most likely find a GW game in progress. But many people, myself included, are working hard to change this.

Allan:
Agreed, but only if you play the current incarnation of the rules, or perhaps one removed. Forget about trying to find Rogue Trader players...

Daric:
Warzone finds converts quickly when you actually get people to play it. Every store has a few Warzone players but they might be harder to dig up.

Allan:
This is distinctly regional. I find it easier to find SG2 or Dirtside II players than Warzone players.

Daric:
SG II is almost impossible to find players for, at least in my experience. 40K has a nice standard set of "tournement rules" that make it easy to play someone you've never met before. Warzone is a little more difficult to come up with a standard as many people play it differently depending on how much they hate 40K.

Allan:
See above. All of the games can be played at most reasonable game conventions. And you can always try to drum up interest amongst your friends.

Daric:
4) Price -- Use what ever figures you like and see above comments on that. But as far as everthing else goes, GW is still the most expensive. The $60 set is all you "really" need to play. But to take advantage of the wealth of other players, you will need Dark Milleneium (around $40 I think) and a Codex for your army ($20) and some may argue your opponents Codex if you are going to hope to beat him. And GW trys to push you to use only GW figs (and the approriate GW fig no less) , which also hurts you wallet. Warzone needs the Basic rules ($20 when I bought mine but a little more now I think). They have two supplements (Both $20 or less. I really can't remember because it seems like nothing compared to all my huge GW purchases) But you really don't need the supplements to play others. Sure everything has some special rules but you can still kill it easy enough if you know your own troops.

Stargrunt only has the basic rules at this point.

Allan:
But for the most part that's all you need (for Stargrunt). It has pretty much everything except aliens. They should come in the Bugs Don't Surf supplement.

Daric:
Finally you should consider the scale, or size, of the games. By this I mean the number of figures you have for an average game. This affects price, obviously, and the flavour of the game. 40K needs the most figs and has the biggest battles. The rules are simplfied for easy of play. (At least theoretically) Warzone and Stargrunt both need maybe half or less of the number of figs for 40K.

Allan:
Sorry, not true with SG2. SG2 needs between 30 and 60 figures per side for a good game. You can get by with 20 (or less) figures per side in Warzone. WH40K and Warzone are essentially skirmish games. SG2 is a tactical game at about one level higher in organization. Funny enough, WH40K seems to use even more vehicles than SG2.

When I played WH40K Rogue Trader, I found that it played best with about 30 figures per side. Unfortunately, people started using it for 100 figures per side game and the whole thing tended to bog down. I remember playing a game at a con that took 1/2 an hour per SIDE to complete. It wasn't my idea of fun. I hear that the current rules are faster for larger battles.

Daric:
Rules are a little more detailed (much more in the case of SGII) so the game takes about as long to play. But they are easier on your wallet, painting time, and storage space.

Allan:
I disagree. The SG2 rules take more things into effect, but the rules are far more elegant. Instead of rolling for each character in a squad, you roll once for the squad as a whole, and once for any support equipment (this is simplified, as you can split the fire of the squad). A good sized SG2 battle can be over in two to three hours of furious maneouvre and fighting. You can usually play two games of SG2 in the time it takes to play one game of WH40K.

Daric:
Everything considered, I recommend Warzone as you best gaming value. It's a good compromise between the flash of 40K and the hardcore StarGrunt II. If you know you are especially attracted to flash or hardcore, then go for the respective system of your choice. None of them are really bad, just different!