Ok I'll bite. The IPA don't offer anything near the level of service that Balpa does unless you wish that service to stop at the end of your nose. Unless your Union have a recognition agreement you'll get zero say in anything to do with pay or conditions as they wont be entitled to bargain on your behalf. Only Balpa has the affiliations and political influence to get anything done externally as substantial contributors to IFALPA and ECA. They were part of the lobbying committee that recently forced the Government to backtrack on its pension reforms. They also have a better knowledge base in the safety arena than many airlines - especially in fatigue and rostering. The days of Balpa being BA centric are long since gone.
The outgoing GenSec was responsible for the current FREA limits we enjoy in the UK. On top of that, 66% of your subscription is subject to tax relief.
The holiday pay currently enjoyed by UK crews was subject to a 7 year battle by Balpa. Only Balpa have the cash reserves to run such a campaign or indeed any industrial campaign these days.
Balpa is not perfect - far from it but they have a good leadership team, their new GenSec has a fearsome reputation and frankly they are the only real show in town.