Lithium. It's nice to have an easily accessible battery compartment, but AAs suck to throw out/recycle and buy. If the Xbox controllers included a rechargeable battery pack for free, I wouldn't complain about them using AA batteries.
Lifetime Sales Predictions
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima