Heavy Rain and Socom are on your exclusive list? I was a HUGE socom fan for PS2 but the ps3 ones are garbage. I really like Uncharted but Gears of War is amazing and you didn't list Fable.
Fable is on the PC as well, it's not a 360 exclusive.
Left 4 Dead is also on PC, not a 360 exclusive.
Heavy Rain is a major game that won many game of the year awards, it's a major release.
PS3 exclusive series:
God of War
Ratchet and Clank
Metal Gear (a spin-off coming out in a while is multi-platform, every other game is PS3 only)
Xbox exclusive series:
Gears of War
I honestly can't think of any other big series on the 360, as Mass Effect series, Fable series and Left 4 Dead series are all on PC (ME on PS3 as well), they aren't 360 exclusive games.
Don't get me wrong, Gears is going to be very fun, but I'd take 13 big name exclusives over the 4 the 360 has if I were forced to make a choice, that too given that the PS3 is a blu-ray player and has free online (not as good quality as 360, but still free works well enough).