Southwest is not a terrible choice. Keep in mind that their redemption rates are strictly based on the fare. Since you claim to book so far out in advance, you will probably be finding the best fares. This means that you will get some decent redemption rates. Many airlines charge the same amount of miles no matter where you are flying domestically. You do not mention where you originate from, but this will also determine your fare to your destination. As an example a flight from SEA -> FLL on Southwest at the beginning of August would be doable for ~36k points, while the same flight SEA -> MIA on UA is 50k points. Of course all points are not generated equally, but since your primary source is points is through credit card spending both of these programs give 1 point per dollar spent, so they actually are equal. It turns out that you would get more free flights using Southwest than United.

Southwest may not fly to MIA, but it does fly to FLL. For my purposes, these are the same airport, but to you it might be like night and day.

The only disadvantages to the Southwest Rapid Rewards program is if you need to book last minute fares (expensive in price, therefore expensive in points) or if you want to travel internationally. Since these are not your goals, I would say that you made a safe choice.