I doubt Bestfares uses Southwest, as Southwest doesn't typically participate with re-sellers/consolidators. I've heard that a lot of Bestfares exclusive fares are through TWA, although they use other airlines as well.

I've participated in numerous online forums for several years, and Bestfares receives mixed reviews from their members. If you are very creative in your airfare searches, I think more often than not, you can get just as good a deal or better, without having to pay a membership fee.

And I've heard some not so pleasant stories about dealing with their customer service people.