The lights are on
If you can deal with the loudness of the 360, I'd say get the Modern Warfare 2 edition 360 w/ 250gb $400 and two controllers. Best deal out there (the elite I got was same price for only 120gb).
As for why you should, pretty much every online game is made with the 360 in mind, the ps3 version tends to get the back end of online/community features and quality. The networking-messages, friends, etc.-all are done much better on XBL. Achievements are easier to notice, they show up where you can actually see them, whereas trophies are easily missed.
The 360 controls are also tighter, which I personally prefer, but that's by preference. Also most cross-platform games have timed-exclusive DLC for 360 versions, so if you plan on getting DLC on the ps3 you most likely will be waiting a few months for it. If you're interested in Netflix the 360 has built in compatibility with it.
If you can't afford the $50 a year price of GOLD then just go ahead and buy a ps3 because a 360 is pretty useless with SILVER. I myself have both, because the ps3 does have some great exclusives.