(Console) Brand name doesn't matter.
Consoles exist to play games, not the other way around. People didn't buy GTA 3 because they bought a PS2. People bought PS2s because it was the only way to play GTA 3. This is why "GTA 4 is on Sony's turf!" doesn't work.
If you're asking about legs, that's simple too.
1) Extremely Frontloaded Sales - This game relies on the "90% sales in the first week" model, so of course legs will drop off.
2) Downmarket customers have reduced access to it - Back with GTA 3, customers accepted paying $300 for a PS2, memory card and the game. For GTA4 you're asking them to pay at least $410. And you're asking them to not buy the Wii, which doesn't have this game.
3) Console userbase - There are twice as many X360s as PS3s in NA.
There is no such thing as a console war. This is the first step to game design.
