B,
receiver use a external battery (4 AA). But because of wight issues I want to use AAA. Even at 900 mAh, does it last longer than 4-AA 650 mAh?
Not necessarily. There is more to battery duration than mAh ratings (even if they are accurate). The poster who recommended higher-capacity AA cells gives good advice.
There is a real good tutorial on NiCd and NiMH batteries at the Hangtime Hobbies (NoBS Batteries) website. It might be worth your time to read it. Pay particular attention to the parts about internal resistance in cells.
Personally, I avoid even AA cells in any airplane. My smallest plane is a .60 size airplane with a Saito .82 and it is flown on 2/3 A cells.
AAA cells airborne, in any but the smallest airplanes, seem to be a bit foolish to me.
Could be wrong.