Sounds like the wireless's also run on AA's, so rechargeable AA's are probably the cheapest overall.

True... but the MS battery will probably be lithium-ion (which isn't available in conventional form factors, because if you use them with the wrong charger they tend to explode). That means it'll have better capacity than rechargeable AAs. I think the recharging cable may also let you play with the wireless controller, using it as a tether, while charging. But it might be that the battery gets removed to be charged, and it's a different cable that provides the tethered functionality.

Either way, I suspect the MS solution will have some advantages. Third party competitors will hopefully spring up, though.