Me personally, I'd take Southwest over American. If a company treats me right, I stick with them. Southwest has consistantly taken care of me. If you ever need to change the flight, its never a problem. They even give you a credit when the new flight is cheaper, and allow you to rebook to save money. When you call thier number, you get a live human being right away. No menus, and hardly ever a hold. (Although thier hold music is a riot)
As for seats... personally, I prefer open seating. I get to choose the seat I want, and the people I sit next to/in front of. (so that I can avoid the kicking baby) When my family flew American and United with the assigned seating, we always got split up. Schedules changed, equipment changed, and before you knew it, I was stuck in the last row of a MD-80 next to the engine, my mother was 3 rows ahead of me, and my father somehow got upgraded to first. Lucky him.