Combo but mostly these 2 things:

1) Domicile/Base. While it can be foolish to pick a regional based on a domicile (some regionals seem to open and close bases fairly regularly) for me it was a "no-brainer". I live somewhere that I'm going to be commuting no matter what, so why not pick an airline that will provide me with the ability to have a commutable schedule and a 1 leg commute? Makes life easier.

2) Movement. Chasing a "quick upgrade" can come back to bite you, but if you go somewhere that is adding airplanes combined with attrition from the top of their list, you should be in a good position.