If you include both x360 and ps3 version, gta 4 will probably sell better, but what we have to think about is japan. Everybody there seems to love ssbb, but correct me if i'm wrong in saying that gta games don't sell well in japan.
Comparason of GTA:SA vs SSBM in Japan
http://vgchartz.com/swlaunch.php?reg1=Japan&game1=Super+Smash+Bros+Melee+-+GC®2=Japan&game2=Grand+Theft+Auto%3A+San+Andreas+-+PS2®3=All&game3=&weeks=150
And since the lines for ssbb seem much bigger then for ssbm, and the fan base is bigger with the wii selling a ton, SSBB will easily outsell it by a large margin in Japan.
If you were to look at charts for the world, you would see that GTA:SA towers over ssbm, however, the fan base of the ps2 had already skyrocketed, unlike the ps3 or 360, but very much like the wii. This leads me to believe that if ssbm was up against one version of the game, it would win, but if x360 and ps3 are combined, it will be a close call.
