Ha, sorry for the thread drift, but good on you for doing your due diligence. As most people have said now, you could probably go to Spirit or Frontier, have a happy career, and retire at 65 feeling you made a good decision.
That being said, if all else is equal and you have/get offers from both, Spirit's ORD base is a little larger, so that's as good a tie breaker as any. And like I said, we have tentative plans to expand in ORD once construction finishes too. Combine that with Spirit's aircraft orders for system-wide growth (plus incredible schedule flexibility), and I'd say come on over!
But hey, everyone I know at Frontier is happy too.