As per Strava:
Mt Hamilton - Official
Distance 18.4 mi
Avg Grade 3.8%
Elev Difference 3,693 ft
Mt. Diablo S. Gate to Summit
Distance 10.2 mi
Avg Grade 5.7%
Elev Difference 3,069 ft
I'd say Hamilton is easier even though it's longer. Has two short descents in the climb that you can use as breaks.
edit; you can just park on the street someplace around Alum Rock Ave.