I believe Spirit is poised to grow, relatively speaking. I've always liked working at smaller companies - even if the money isn't as good as a major. Plus they are east coast headquartered.
Why back to 121? 135 is a lot more problematic in terms of trying to have a life than it was when I did it several years ago. And the increase in regulatory oversight, SMS, etc. is doing nothing to combat the rampant issues of rest time abuse and fatigue. I'll avoid it if I can - I find it extremely stressful.
I fly Part 91 now, but the gig is not quite full time and the pay suffers; corporate jobs, even for someone very motivated/knowledgeable, can be elusive. If it paid better I'd stay put. It's a mid-size jet co-captain position for a family, the schedule is fantastic, and it's low stress.