Well, I guess my question is what is there to gain by being in another "ultra" status program if you do not really use them? The only thing I can see you maybe shoot to finish up is AA 2MM. You can probably shoot for DL or AA ultra status, but....
Ultimately, I guess it depends on your travel patterns and whether you could benefit off of an additional ultra status program...
Since you fly a lot of full fare, as people have mentioned, CO has always gotten good reviews for its domestic F.