I can see possibilities with BALPA aligning with a much larger union. Part of the reason BALPA dues are high relative to other unions is due to it's lack of scale.
Whilst in some ways I admire BASSA, I would be very weary about being attached directly to it. Pilots are very different people from the majority of cabin crew and our interests are not always aligned: witness the current goings on in BA.
Agree with al446 that IPA certainly isn't the way to go. Completely the wrong direction in fact.