When I was looking for my first 121 job, my best options were skywest and spirit. I didn't even consider frontier because they were so grossly unrealistic that they were still requiring a 2 year payback training contract. And the writing was on the wall regarding the contract fiasco. Who wants to stick their nose into that mess when skywest hours look just as good (or better) on a resume? Get your 121 PIC time any way you can. At this time (and for the last couple of years in fact) Frontier is not a move "up" in any way.
I don't think anyone in my SWA class came from frontier. We had someone from spirit and everyone else was a real mix of almost every possible career pathway, corporate, military, regional, etc etc.
Go to Frontier if that's your idea of a great career final destination, but the real question is always going to be why use an offset aimpoint when the normal career progression is still proven to be the most reliable way up the food chain. If I had to do it again or if SWA suddenly went away and I was on the street tomorrow, I'd happily choose skywest over Frontier like a whole bunch of my friends did. There is real value and security in working for a proven *company*. Find a regional that gets you PIC the fastest, and grind out your hours there instead of trying for a half-step "up" to frontier which in reality is just a side track off the proven career path.
Just look at Spirit and see what happens when a company bites the bullet and agrees to a big boy contract. They're printing money over there, even though it took 2 quarters of reporting to investors that growth was on hold due to the contract negotiations. Once Spirit management decided they'd rather pay up than have to tell their investors AGAIN that they sucked at their job, the contract was a done deal and the company could keep moving forward. Frontier doesn't seem to have those pressures, happy to keep limping along on the strength of cancelled vacations and deferred MX.