I would spend some time in all these neighborhoods and decide. The mission is also a great place. Good weather, centrally located, and lots of bars and restaurants
If you wanted a mellower vibe, but still really close to Haight Street, you might want to consider Cole Valley. Just a few blocks away, but without the intensity. If you like the urban, all the time vibe, I agree North Beach is also a great place for you to check out.
SOMA is different -- loft living, and much lower concentration of nightlife, but easy to hop on bart. Nothing to sneeze at that you'd also have the Embarcadero & Bay right outside your window!
Janis and I frequent SF and love just about everything everywhere. We are long distance walkers and love walking city streets and sandy beaches. We once walked from Fishermans Warf to The Haight and even threw in the "Crookedist Little Street In The World" for an extra challenge.
It's one of our most favorite walking cities in the world as I'm commenting on this question from Paris our most favorite walking city of all time.
I'm a bit bigger fan of Cole Valley, which is adjacent to the Haight but a little less crowded and cleaner. SoMa is nice, although it has more of an industrial feel than a neighborhood feel. But the weather is good, and there are plenty of bars and restaurants. North Beach does have a bridge-and-tunnel crowd (and tourists) but the deal-breaker for me with North Beach is parking - if you must have a car in that neighborhood, a garage is a must. In that vein, street parking in the Haight isn't very good either.
Hope that helps!
Better Homes & Gardens/Mason-McDuffie Real Estate
2200 Union Street
San Francisco, CA 94123