Depends on age I think, but 2-3 year wait looks like a no-brainer.
But with all of the legacy retirements seniority progression on a legacy narrowbody fleet is likely to beat any progression at a ULCC in the mid-term.
Obviously you should apply OTS to the big three as well, and go immediately to any that call.
There's always a slight risk that something bad will happen, and stop all hiring. But if it's bad enough to cancel out all of the short-term legacy retirements it will be bad enough that ULCC might furlough. In that case might be better off at the regional anyway.