GT4 is the only noteworthy title that is not available for xbox. Since personally I think Forza much better, I could care less.
I’ve owned a ps2 and 20 retail games (no burned, no mods) and I sold it because it did nothing but collect dust after I bought an xbox.
summary:
graphics: xbox > ps2
online play: xbox > ps2
game titles: xbox > ps2
ease of mod-abilty: xbox > ps2
and let me just add that IMHO: Halo 2 + Xbox Live > *
that is all. 
my $0.02.