We should not compare these two games. They concern different genres. I game in Call of Duty Modern Warfare2. It is good game but it rectilinear and quickly bothers. Thanks developers that they in this game have built in possibility to play on the Internet. Just it also does WM2 interesting.
Now I play GTA SA-MP more but sometimes I come in WM2
MW2 is a piece of garbage first person shooter. It just was massively advertised, so noobs around the corners that just found out that Xboxs existed started buying it. There are MANY FPS that are way better than MW2. Battlefield series for instinct, I played each one since Battlefield 1942 (still fun after 8 fucking years). Battlefield > MW2 by far. Even the Medal of Honor series and Counter strike are superior...
But w/e, I vote SAMP because of it's versatility (although you shouldn't compare an fps and a sandbox type game).