Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
In your situation, I would probably go with Southwest, if you don't mind flying SFO-ATL-SFO (or making a connection out of SJC). Southwest offers SJC-AUS nonstop service, which would come in handy for you.
It's tough to compare the different FFPs, as DL and UA use a zone-based redemption scheme, whereas WN I think uses a cost-based redemption scheme. But if you end up earning enough Rapid Rewards points on Southwest to qualify for the Companion Pass, that should easily trump both DL and UA for domestic redemptions.