Thanks, all.

Virgin miles definitely a plus.

United has dreadful service between SFO and MIA. Otherwise, it's my usual carrier for the U.S. (Alaska for Mexico and Virgin for UK). However, I note National has very attractive, even affordable, first-class rates and that interests me, especially considering the recent difficulties in getting confirmed UA upgrades at time-of-booking. With fewer flights, there's less availability. A round-trip $900 first-class rate SFO-MIA looks pretty good to me.