Xbox LIVE is superior to PlayStation Network. I'm not biased (I own both systems), but paying less than three dollars per month (you can find 13 month cards on ebay for 40, I get them for 30 because I ebay snipe lots haha). Even if you buy it retail at 55 (simplified the tax to 10%, I know it isn't) 55/13 = 4.23 every month. That's like one less happy meal every month, lol. Not a big deal at all. One HUGE problem PS3 has is its interface...it's ugly, confusing, and tedious. On the 360 you can easily jump into friends' games, create a party, start a chat, and even jump from game to game. This simplicity isn't available on PS3 and will remain a problem until they decide to release a better interface (hint: never). As far as exclusives go:
360:
Gears series (Gears 2)
Halo series (Halo 3, Halo Wars, whatever else)
Fable II
Kameo
Too Human
Left 4 Dead (also on PC)
The Maw (also on PC)
Castle Crashers
[more I don't feel like listing]
PS3:
Resistance series (Resistance 2)
Ratchet & Clank series (Tools of Destruction)
Metal Gear Solid 4
InFamous (comes out June)
Uncharted series (Uncharted 2 comes out holiday season)
LittleBigPlanet
Killzone 2
it's important to note some Downloadable Content is exclusive to Xbox 360, Fallout 3 (3 DLC packs) Grand Theft Auto IV (2 expansions) are worthy of mention for this