As Cinco said, if the merger goes through you will be in the same situation in regards to seniority regardless of which company you start with, so your best option is to take the first available training class.

That said, if the merger goes through, JetBlue will add very senior pilots to both Florida bases. You will of course remain in your base if you are there before the merger, but the time to captain will increase a bit. With 15 years left, an upgrade is not out of the picture given their stated expansion, other pilots leaving, and retirements. However you might find for lifestyle purposes being a senior first officer who can choose his schedule and vacation weeks preferable to a junior captain on reserve through his sixties. It’s impossible to know what the situation will look like a decade from now, or even 2-3 years from now with a lot of merger unknowns. But it would be prudent to run financial numbers based on the new JetBlue FO pay scale to see if that would be a tenable situation for you.