I reaklly think that's a tough choice, but here's one question that might help, if you have a slower system and video card, you'll hate SWAT, I've got a PII-300 and a voodoo3 and SWAT runs like a slide-show at times ,especially in the Carlyle hotel and at the convention centre (the one where they hold E3).
though I liked SWAT's playability a lot more than RS's, there's a dynamic campaign (IE: missions are in different order, random terrorist placement). Also it has great AI, your teammates will move around on their own, terrorists will know when to surrender (and talk too). Also the Graphics are incredible (though it's like a slideshow at times), there's mirrors, halos when you turn on your flashlight, and some other cool effects.
and the options are cool too, like you have to use a toolkit to open a locked door (or use C4 or a breaching shotgun), there's the opti-wand (a video camera on a stick for looking around corners).
of course, I don't find there's very much replayability, there's no terrorist hunt, no multiplayer (yet), no sniper rifles, a limited choice of weapons.
on the other hand, RS is pretty good too. it has a pretty good story IMHO (Wierd how both games seem to involve those suitcase nukes), and it's a little more simple compared to SWAT (that's a good kind of simple). There's cooler weapons (I love the M-14 and the Barrett), sniper support (unfortunately only a few levels where you can use it), Multiplayer (plus a cool multi-only map called Bunkers). it also has a level editor, a weapon editor, it's easily modifiable (no more using the MP5 model for a M249 anymore, and you can put in an actual grenade launcher), some REALLY cool missions (particularly the two missions in that russian radar station), it has some great replayability with the different modes you can pick (Terrorist hunt).
