Not sure about culture, but pilots seem to be happy at Republic. Route and overnights are probably a little more diverse at Republic because you are flying for all three code-shares and staying at many different cities based on who you are flying for that trip.

As far as non-reving, Envoy has nice priority on AA. But having non-rev benefits for the big three does leave you a whole lot of options when trying to get somewhere, even if you are low on all three.