First of all how can't really compare killzone 2 and halo 3 because one game is out and one gam still has a year of developement. so yah dont compare them jsut yet because we didn't hear too mcuh news a bout the game yet but from the previews its fantastic!
As for Timeshift an above user posted its on both the 360 and PS3 it jsut the PS3 got delayed for a few months so its coming around december for the PS3.
As for online play LIVE does have more features than PSN but PSn is free so you can really complain on something that you get for free. While live you pay $50 per year. ITs not really worth it since their aren't really any major differences between the two. Just LIVE as a bit more features such as voice chat in all game and a larger buddy list ect nothing to major. But PSN and LIVe both have co-op, death match and all that other great stuff.
Hardware reliablity wise PS3 is much better. The 360 has a severe problem of over heating which results in the red ring s of death will the ps3 on the other had has NO hardware problems. Also you may hear people say there are newer 360's that dont have this problem but they are lying to you the problem is still there.
Games wise both systems have good line ups. But i fine that the PS3 has more variety in there games. The 360 has mostly shooters while the PS3 has shooters,RPG's , platforms and many other genres.
Here's a quick run down of games on both consoles that will be great
also ill only put exclusives titles)(mulitplatfor games are like assassins creed and call of duty 4 ect) some are released some are coming in 08
Xbox-
gears of war
bioshock
halo 3
dead rising
mass effect
ninja gaiden 2
lost oddessy
banjo 3
alan wake
forza 2
PS3
resistance
ratchet and clank
resistance 2
uncharted
jak 4
Warhawk
heavenly sword
folklore
infamous
metal gear solid 4
little big planet
heavy rain
tekken 6
star ocean 4
killzone 2
eight days
socom 5
grand thurismo 5
HOME(online interacting service)
Lastly the only difference you'll find in multiplatform games is the the 360 has achievements. But the PS3 is getting thropies soon(around march 08) and the PS3 uses 6 axis controlls in certain games
Overall i prefer the PS3 over the 360 in every catagory.
Better games
Better online
etter build console
Verdict PS3