I just looked up the hostels on the map. The Ellis St. hostel is very close to Market st. which is the core of the Muni Light Rail, and also a few blocks from Van Ness which is one of the main bus corridors, and from Union Sq. which is the main shopping district in SF. It's also close to the opera and symphony if you like that kind of thing. It's also a reasonable walk to SOMA (Modern Art museum, trendy restaurants, night clubs) Castro, Chinatown, Hayes Valley, and Lower Haight, which are all cool neighborhoods. The immediate neighborhood off Ellis can get a little sketchy if you go the wrong way - e.g., you're on the edge of the Tenderloin which is not the best place for a single woman on her own, but if you've got good city instincts it should be OK. The hostel in the Presidio is going to be in a very beautiful park-like location, though is not as close to downtown stuff.
I'd offer to play SF tourguide as I lived in an apartment in the city for most of the last 15 years, but just bought a house about 20 mi south for family reasons. I could be up for climbing or hiking or just drinking some beer with you and other SPers on weekends, however.
+1 for Luciano's suggestion of going to SF, touring the coast, and then setting up basecamp on the East Side of the Sierra. Bishop has lots to do for climbers and hikers. Mammoth also has a lot of options and - because it is a ski resort in the winter - there are a lot of condos available for weekly rental, often for less than you can stay in a motel, especially if you get a few friends together.
If you find your itinerary filling up with so many things to do in CA that you'd prefer to stay there I'll make the trip. This is your vacation and you must plan to do everything you want to!
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