While I see you already eliminated it, AMH is obviously a much shorter run so I kind of vote for it for that alone. It's actually a really solid, mean machine, although it does have a few rough edges on it still being their first game.
BM66 is okay. I've heard others talk about how much they love it now, but to me I just never get into the playfield design.
Maiden is a great game, but one you'll be able to get new for probably another year at least. You could get, play, enjoy and move on the AMH and then pick up a Maiden without much issue if you chose to within a year or so. If for some reason I was being forced to make this exact decision, that's exactly what I would do.