Been a member of both ...
The success of a union depends more on the intelligence and experience of the local council and the cohesiveness of the pilot group than the particular union HQ. Both have strengths and both have weaknesses. The choice would also depend on whether you are a regional, a national, a major, or a freight/charter company. Generally speaking ALPA tends to be a better advocate for the majors. Generally. Teamsters may have an advantage with a freight company when the truckers honor a pilot strike. Of course, FedEx is ALPA, and they have a reasonable contract, so being the size of a major may favor the ALPA playbook for them.
But, again, you need a smart EXCO/MEC and a unified pilot group to have any real success with whichever union you end up with.

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