I bought my 360 for gears of war originally and I never regretted it, so far i now own and love:

Gears of War
Halo 3
Orange Box
Darkness
PGR3
PGR4
Forza 2
GTA4
Guitar Hero 2 & 3
COD3
COD4

I never got into SA so I cannot compare but I can say GTA4 is a great game where even the shooting elements feel polished enough for them to play as games in their own right as the aiming system is spot on and there's plently of mini missions to be pottering on with so you'll get a lot of gameing hours out of the single player campaign alone.

You could just wait till it comes out on the PC.

After a few hours on GTA IV I felt as though it was as though the game took 3 steps forward and one step back. What is new in the game is very, very good, whereas what is lost from SA isn't all that bad. Overall the changes are beneficial to the game.

I bought my 360 for Halo 3 originally, then quickly added Bioshock, Gears of War, and Dead Rising. That last is also a great sandbox game, with the only downside being the time limit.

Dead Rising also has a free demo.

Oh word up.  Oops.

I wish my friend with a 360 would like... hook it up to the internet so I could play it at his house then.

buy a 360. there are tons of awesome games!

GTA IV is worth it alone. You will get at least 60 hours out of this game, and in the current circumstanes, that is equivalent to like 5 or 6 games. And it will be the most fun you have ever had. It is actually true that with one city it is pointless having planes. A chopter will take you around to see the city from the air, and you can always get a taxi to take you instantly anywhere

SA> GTA 4

 Unless that game is Shenmue 3.