We use Southwest for our domestic flights because their prices alone seem to never be beat. We use our points a lot as well, but when we do have to pay, they are far below their competitors. They do not show on most travel sites with the other airlines like Orbitz, so you will need to go to their site directly to compare costs HERE.
Although they do not allow you to reserve specific seats, there are opportunities for cheap upgrades to get priority seating. For instance, you may be in group B, but for ~$15 more each trip, you can move up to group A and choose your seat before that second group. If you have to have a window seat, that upgrade is probably a good idea.
A big perk to using Southwest is their luggage process. They allow two checked bags for free as well as two carry-ons. Unless you have a perk with a different carrier, this can really add up. Even one bag could cost you around $50 round trip on another airline on top of the higher flight cost. Therefore, you have to take those extra fees into account for your overall travel costs. Check them out for your next trip, and you could ride in style on one of these bad boys…