I have been a member of the IPA for quite a few years now, and also joined the IPF when it was up and running. I can't compare it to BALPA as I have never been a member, but the help available from the IPF to you (legal representation etc, should you ever need it) should have you covered. (I wonder how all those CityFlyer chaps felt represented by BALPA when they 'merged' into BA).
Plus, the IPA/IPF in nowhere as expensive.
I have not needed them in anger, but every time I did phone them up I received quick and professional help, very friendly.