Sort of thinking aloud here. Anyhow, when considering buying a game, these bits usually run through my head:

Is this game going to be hard to find in the future? If the answer is yes, I'm more likely to buy it sooner rather than later. For example, I bought Virtua Fighter 5: Online based on my horrific experiences trying to get copies of Virtua Fighter 4 and Virtua Fighter 4: Evolution. I bought Katamari Damanci - no review, never having heard of it - on my friend's recommendation because it was in short supply (and I implicity trusted his taste in games). Big, popular games? I don't worry about those. I'll find them easy enough even a year or two later.

Is there a particular buy to support this game/version? I waited for the PC version of Devil May Cry 4 just to support Capcom's PC efforts. I bought Galactic Civilizations 2 because I support Stardock and their views on DRM. If my computer could run it I would've bought the new Prince of Persia game to show Ubisoft I support them and their experiment to make it DRM-free. SNK? I've almost always got money for them :)

Will I get my money's worth? I've discovered a while ago certain types of games are worth more or less to me based on interest. A good fighting game that I'll play regularly? Full price is fine. An RPG, on the otherhand, I won't pay full price for anymore. Why? Because I don't replay them, won't go back and find everything, etc. That's a lot of the value gone for me - why buy brand new when I can rent once, twice, three times at most and beat? I beat KOTOR and KOTOR 2 in under two days each (marathon playing). Platformers? I almost never finish them, so paying full price (or even half-price) is out of the question. Ditto for FPS-games, really. No sports game is worth my money except for a fun, arcade-****baseball game, wrestling or MMA. So on and so forth.

Will I keep this game? This is a new addition, actually. Since I've got a PC and plan to get a 360 next month, I have to wonder now "will I keep the game?" I can't trade-in PC games, after all. But PC games can get mods, user-generated content, I can use FRAPS and make game-movies, and a whole host of other things. However, with a console I don't have to worry about getting updates (automatic), driver issues, trying to figure out obscure settings to get something to work, worrying about performance, etc. Plus I can grab a console, throw it into a bag with controllers and games, and go to a friend's house, easy. No can do with my PC. So, yeah, figuring out which version I want can be tricky in that regard.

No, this isn't comprehensive, and I don't think of every thing every time. But, this happens often enough. How do you decide what you're going to buy?