Much evidence has been aired on Repa of IALPA supporting numerous court cases involving Ryanair employees.
I don't recall reading about one case where BALPA was in the dock on behalf of a member.
With this in mind, who do you prefer on your side ?
If you have a UK contract however, you are probably stuck with the BALPA option. At least you can console yourself that you are helping to contribute to the snail like march towards recognition.