Recently I've been re-visiting the 'union or non-union?' question (because possible union contracts are popping up more frequently) at least for stage work (non-union for on-camera is my only good option for now.) I solicited advice from well-rooted Philadelphia actors and producers as well as from mentors in Houston and Portland. A Philadelphia producer managed to put my choice most succinctly: if I want to establish myself as trustworthy in Philadelphia I should stay non-union until I have more exposure; if I want to focus on regional work, which casts mostly out of the AEA center in NYC, then I should go union. The choice depends on which priority is greater.
So, for the next little while, at least, I intend to stay non-union (unless something too-too tempting comes up) to fully establish myself here, in Philly. Once that feels reasonably done I'll seek out AEA. The pay may be crappy non-union, but the roles are good, I like the Philadelphia scene a whole lot, and I want to put down roots. Also, I can build both a local and national career better by establishing myself first in Philly than I can by establishing myself first regionally.
(I think I have greater ambition for doing regional theater than do many local actors though I'm not sure why. I enjoy the travel and living in dorm-like conditions for a few weeks at a time. But of course I don't have a family or day job to worry about.)
I'd rather be union, no question, but I can wait a bit. I feel good about my career at this point no matter what. The important thing now is just doing the work.
Thanks especially to Dan, Brenna, Jan, and Jack for the well-informed advice.