If the flights are still available. I usually end up booking on United (for an SFO -> somewhere east of the Mississippi flight) if I have to do something last-minute, because I can often still get direct or 1-stop flights to my destination, while Southwest will often have only 2-stop+ ones that take 12 hours and cost more.
I find that very often United fares are lower than Southwest anyway. Perhaps it's because SFO is their hub.
Virgin is another surprise - a couple times I've ended up getting SFO -> BOS or SFO -> JFK flights that are cheaper than anything else on Virgin America, even though it's supposedly a "premium" airline. I think they may be offering specials to build up repeat customers on the route, though.
I find that very often United fares are lower than Southwest anyway. Perhaps it's because SFO is their hub.
Virgin is another surprise - a couple times I've ended up getting SFO -> BOS or SFO -> JFK flights that are cheaper than anything else on Virgin America, even though it's supposedly a "premium" airline. I think they may be offering specials to build up repeat customers on the route, though.