Go with the first job offer. Jump ship if it gets you in base sooner/longer, or if you really think the grass is or will be greener on the other side.
AA is the only legacy I didn't apply to. F9 is the only ULCC I didn't apply to. AA because every time I jumpseated on AA the crews seemed really disgruntled and unhappy, and F9 because they're run by an investor group with no desire to do anything but squeeze and squeeze. With all the options out there, I simply didn't think I needed to put myself into either of those environments. No place is perfect but at the time both of those just seemed toxic and unhealthy. AA's persistently high debt ratio and F9's cancelling of vacations during contract negotiations tell me my concerns were not unjustified.