Justin, They are both lovely, but the attraction of Aonach Beag is that it forms part of a ridge of 4 Munros - Beinn Eibhinn, Aonach Beag, Gael Charn, Carn Dearg,- on the north side of the bealach and is a bit easier underfoot I think. It also gives more of a straight line than Ben Alder and Beinn Bheoil. You'll get great views of Ben Alder, weather permitting of course,

I'd agree with Kirsten. There are better routes up Ben Alder. The ridge will give you a lovely day and you could continue over the Fara the next day. Just a shame that Culra bothy is closed.