I would take the first one that offered you the job. You can work there, see how it goes and keep applying to the other one if you don't like it.
Airlines are funny. A candidate can look like the perfect fit and not get hired.
SW is a great company. I have friends that left here (US) have not looked back even though they are still a F/O there and could be a captain here. Will they change their mind now that it looks like a merger will go through? Who knows. They say it was like Piedmont used to be, they enjoy going to work while they didn't in the darkest days of US.
There is going to be a couple years of rough edges to knock off at the new AA. Sounds like SW has a few with AT.
I have enjoyed the variety of flying I've done here, from 8 legs a day on the F-28 to trans-Atlantic on the 767. I have over 8,000 hours on the 737 and enjoyed it when I flew it, but after the 767 and A320 I never want to fly another one. I never flew the NG, and even though I'm sure it's better, it's still a 737.
Good luck!