No contest. The IPA is not a union and for all it's good work in certain areas has no recognition agreements with any airlines as far as I know. That is the crux of it so in other words they are toothless.
The IPF has some sort of tie up with Ashtrays I believe but this is very much a one off.
If I'm honest I am not over enamoured with Balpa as an organisation, I have served on a couple of CCs, but like BSD I am a member as an insurance policy first. anything else they can get me with Ts & Cs is a bonus.
Sbird 800, my advice, ask yourself what you want the union for and what you get for your money, then come to a decision.