Theres an easy way, an efficient way and a hard way to do things. Microsoft tends towards the efficient model.
"With 2xmsaa, somewhere in PS3 memory must reside a 2560x720 buffer to render to as well as the regular 1280x720 buffer to resolve out to. The memory for both of these must be available somewhere in the 512mb of ram. On 360 you do not need the 2560x720 buffer in the 512mb of ram, you render msaa to edram in tiles, and it gets resolved out to the 1280x720 buffer in the 512mb of ram. That's a big memory penalty on the PS3. Likewise, rendering PS3 particles to a 1/4 size memory buffer means that somewhere in memory must exist a 640x360 buffer to render to. If you are lucky and can reuse a buffer from elsewhere then cool, if not then that's more memory eaten. No need for that on 360, you render particles straight to edram at full res. Etc, etc. Put it this way, if it ever does become only a 6mb difference then I'll have a party at my LA pad and you're all invited. For now though, it's not Memory is what you make of it in any case, God of War looked amazing on a 32mb PS2."
Just one more reason why memory is an issue for more the PS3 and less the Xbox 360.