If it's at all convenient to your origin, 99% of the time, 280 is the better choice to San Francisco from the south bay or peninsula compared to 101. IMHO, taking 101 is always a roll of the dice. Drivers I find are generally slower, less reasonable as far as removing themselves from the fast lane, and the freeway is generally just more congested.

From the south bay, e.g. Saratoga Ave, there are indeed HOV lanes all the way to El Monte Ave in Los Altos and most of the time that's the only area they'll be needed anyway.