I agree, F is worth the extra $$ if you can afford it. Even SFO-LAS.

If you're willing to chance it, there are 24 hour upgrades ($50 each SFO-LAS) when seats are available. These tend to go quickly (if available at all), so there is a good chance you will miss out.

There are some details under the FAQ's: http://flyertalk.com/forum/showthread.php?t=838255 (see: 'Q: Can you get a standby upgrade to first class?')