Quite a different types of games really. Stalker is... well Stalker, and Arma2 an open-world infantry combat simulator with supporting elements like armor, air and artillery modeled to some degree. It's a pretty direct sequel to Operation Flashpoint(no, not the Dragon one) and Arma1, if you've ever played those. Dozens of countries use VBS, its military brother, for training, and this also shows in the "civvy version": It's pretty realistic in many ways, but also takes shortcuts in areas which you'd expect to be handled properly after playing normal tubeshooters like COD and MOH. Arrowhead improved this situation a lot, but it is still there. Normally single player campaigns are the weak point of BIS games, and multiplayer is a big strong one. I think the amount of players there is theoretically unlimited, but practically the max is around 120. There are no set gamemodes, everything is up to the missiondesigners. A mission editor is included with the game. I did find the original Arma2 campaign decent enough, if somewhat slow-paced at points. Haven't touched Arrowhead's campaign so I can't say for that. I went straight to multiplayer with that one.
The default world of Arma2 is quite a lot bigger than anything you see in Stalker, around 220 km2. Arrowhead's Takistan is slightly smaller I think. Modding has always been a strong point of the series, and Arma2 is no exception. Dev tools were released shortly after the game itself, and the game itself is built on top of modding. Even all the original content is actually mods. There's a ton of mods out there varying from increasing realism to WW2 to Star Wars. There also used to be a Stalkerish MP mission around.
The OFP-series has taken most of my gaming time ever since the original OFP demo was released in 1999, so it can't be that bad
edit: And now I see it's Spike who's asking. Nvm, you are familiar with Arma already
