Have you opted in to the Aviate flow or are you in any way involved in Aviate besides simply flying for an Aviate regional? If so, you might be forced into staying if you want to have a chance at United. Once you're in the Aviate program, if you withdraw you're essentially guaranteed to never get a job at United.

I'm not trying to scare you or anything, I just want you to be aware of that so you don't make a decision you could possibly regret later. If you're not part of Aviate, then I definitely think the above advice is worth pursuing. Last time I was at TK (the flight training center) I met a handful of former Spirit pilots and maybe four or five former Alaska pilots, so I think going to F9 would probably be a wise move.