I'll say this as someone who owns both games--Saints Row 2 is my pick.
Why?
[+]TONS of stuff to do, even after you beat the main story
[+]It is essentially a humorous version of GTA IV, minus Roman (Though, minus Roman should be another plus...)
[+]Memorable characters
[+]Money is NOT necessary in this game, it just makes things slightly easier
However, there were some annoying bits:
[-]The graphics were about...20% better than Saints Row's graphics.
[-]The game occasionally froze on my 360, even after it being installed to my HD.
I'm not going to dissect Grand Theft Auto IV, as I really don't see a need to; it has all the things Grand Theft Auto 3 had, and a similar story--except, this time, you're Russian. Though, it did have Katt Williams, which gives it +30% in my book.
Basically, the game speaks for itself. Both games are good, however... I prefer Saints Row 2's lightheartedness to Grand Theft Auto IV's melodrama. Both games are quite good, and quite worth playing, though.
