While it's been many years since I rode up Hamilton (snow on the summit!), and I've yet to ride up Diablo,
The climb from Fairfax to Mt Tam is pretty awesome. Lakes, osprey, redwoods, little traffic, views of the Farallons, and view of the golden gate/city from the summit. You can get as much workout as you want, too.
I don't think it beats the view of Half Dome and yosemite valley that you can get on a clear winter day from Diablo, and while I live in Fairfax, I don't think I'm totally biased,
you won't be sorry if you check it out.
(Mill Valley side= more traffic. Post ride beers at Gestalt Haus or Iron Springs Pub in Fairfax)
For bonus climbing, don't miss the far end of Bolinas/Fairfax Rd.- a steady 1600' in 4+ miles.
http://ridewithgps.com/routes/4236269