As in which association you should make enquiries with? In reality you need to do your homework and do both.
Which do I personally think is able to serve you more effectively? AIPA. It's bigger, got deeper pockets, is used to representing crew engaged in longhaul international ops, has a better idea of the 'big picture' (as evidenced by recent AFAP actions), etc.
Note: I'm a 15 year AIPA member and have never been formally involved at any level with AFAP. I've never served on the AIPA COM (although that may change in the next year or 2). My knowledge on the AFAP is based upon reports on PPRUNE, contributions by Lawrie Cox both here and in the wider media, and discussions with colleagues who have been members of the AFAP.