The pilots at UPS found that the Teamsters airline division as it existed in the early 90's was not working for us. This was in large part because the Teamsters national decided that we as a group did not have the experience to run our own local, they invalidated our elction of representatives. The pilots who had been elected to offices to represent us then took a huge risk and started the in house organization, both the Teamsters and UPS fought this.
The formation of the IPA required a huge effort on the part of every pilot on the property, money contributions, letting the guys starting the union know they where backed, and when a single line pilot was cut from the pack to be sacrificed by management letting the company know that we as a group of pilots would not tolerate this ( on a couple of occasions when a pilot was being berated by management they where actually surrounded by line pilots in the area which brought this practice to a stop).
Look at how soon you as a group will be allowed to run your own local, how much initial support you will have from the national, who you have willing to step up and take on the positons that require a very high level of courage, look at your fellow line pilots and see if you have a group willing to go toe to toe with the company. This is what you as a union and a pilot group must have, if you don't no union national or in house will accomplish a thing. If you have a group who wants to sit back and not participate on a daily basis , volunteer for committees, give up time , etc you will not reach the objectives. You will find that the pilots who complain the most about the union are the ones who are willing to contribute the least if at all or curry favor with management.
If we did not have an in house union at UPS I would favor being ALPA, while I have walked the picket with the Teamsters and been represented by them at two different companies, I think that ALPA has a better understanding of the needs of pilots.
Either is better than not having one and neither is any good without the resolve and support of the pilots they represent.

Good Luck