Home Owner, Anza Vista, San Francisco, CA
BART is almost always faster, so much so that even if it's a longer walk to the station you will probably be better off with BART. The trains are underground for the entire journey so they do not have to compete with surface traffic like Muni does, and there are only a few stops on BART compared to many on Muni.
The best resource for finding out which public transit route to take for your specific circumstances is the official Trip Planner: http://tripplanner.transit.511.org
Under "Additional Options" you can force it to include or exclude BART or Muni, if the default settings don't give you an option for each.