Welcome to Flyertalk, cdhbfcp!

I'd say probably the main reason(s) for you to go with Southwest would be one or more of the following:

1. You change/cancel flights often. On Southwest this is free, on legacies not.

2. You have a shot at the Companion Pass (which you will with your frequency of travel), which gives you free flights for a Companion on every Southwest flight you take. This could save thousands if you have a spouse, girl/boyfriend, partner, business associate, child, whomever to take on domestic flights.

3. You want to fly on all 737s (no tiny jets).

Or you could do a mix of airlines (depending on destinations, non-stops, schedule, etc.) and achieve high status in both. My guess is that this would be the optimal solution for you.

Visit the various forums here and start reading.