The quick answer is......Whoever calls first as long as those two companies are still an attractive place to be when you get to that point.
If Im right and you haven't even started at the regionals, military, cargo, or charter, you have a lot of time and a couple of economic cycles to see how things shake out.

Things can and will be different by then.

It certainly doesn't hurt to ask questions. APC has a lot of info on the forums to read through, so grab a beer and do some research.
My advice is don't go chasing money or seniority because you will only sacrifice seniority and income while doing so. I was on the j/s for Southwest around 2007-08 and met a new fo with southwest who bailed on Delta because they were in/just exiting bankruptcy to go work for Southwest. Southwest is a great co, but looking back, seems like a bad move. That same guy lost good pay AND seniority with Delta.

Please, for all of humanity, don't work for Gojet or Mesa and you will be fine
Good luck.