In today's pilot marketplace , lateral moves seem to be more prevalent. Bases are important for sure, but grooming yourself for the eventual move to a major is far more important than a short time at a base you don't want to be. Recruiters at the regionals(I was one at eagle years ago), are not going to question your loyalty, but applaud your decision to advance your career . Regional's know you are trying to get in and get out . If I were making a lateral move , everything else being close to equal , I would go where there is a flow. The upgrade time/flow numbers espoused by envoy may not be totally accurate , but there are a few hundred captains leaving every year, as long as hiring continues at AA. Although not likely to be hired off the street to AA , (from envoy),nothing precludes you from applying to everyone else. Having a flow number to AA, weather it is 6 years away or 10, is better (in my opinion), than having to apply and hope like the other 18,000 regional guys. While I won't comment on the good/ bad argument for envoy, most regionals are very similar in pay/working condition , or if they aren't , they probably will be soon.
Good luck with your decision .
Joe