I was under the impression that the primary benefit of Cell-based MLAA was that developers by and large have plenty of CPU power to spare on Cell, thus allowing for the implementation of AA without taking a hit in other areas. I'm not sure how a direct comparison to a GPU-based solution is very meaningful here.
Edit:
Ah, it seems WereKitten already covered this. It provides an excellent alternative to MSAA for 360 developers either way.