Northern California - San Francisco - MTB or Road

My wife and I are heading up to SF from Pasadena for a long weekend in early December. She will be there for a conference the whole week and I will drive up to meet her on Thursday night. I will have all day Friday to do a bike ride. We will likely stay in the same place we always do in China Town near City Lights Bookstore.

Should I bring the MTB or road bike?

I would like to get in a nice long ride whatever I do. 70-80 miles road or 20-30 dirt. One thought for road would be to leave the hotel and bike across the bridge and up to Point Reyes. Traffic doesn't bother me too much and I don't get lost easily (finishing my Masters in Geography and a cartographer by profession). I can ride most dirt trails so long as there are no jumps or anything I need to huck off. The more technical otherwise the better.

Thanks in advance for any assistance provided.

Pt Reyes would be a great ride providing the weather cooperates.
http://www.velogirls.com/resources/routes.php#north
http://www.velogirls.com/resources/routedocs/nicasio_trainingrideV2.pdf

ride ROAD! there are some great spots here for mountain biking, but if you bring a road bike you can ride right over the golden gate bridge and into beautiful mountains in the middle of quite nature. might i suggest the alpine dam loop? google it.. there are some pictures of the nature in marin on my site here:
baildownworld.blogspot.com

we have some pretty damned amazing road riding here.

I'd have to vote for Road too. For MTBing there is some stuff that is ride-able from the city but most of it is driving distance.

I would have to go Road! Here's the Alpine Dam Loop Route:

http://www.touchstoneclimbing.com/pdf/alpine.pdf

There are plenty of great MTB rides you can start with-in SF. A popular favorite is to ride to the Golden Gate Bridge, across the bridge and into the Marin Headlands. Since you'll be here over the weekend, you can take the western bikes-only walkway across the bridge and not have to weave in and out of tourists. After you get off the bridge, head through the parking lot and turn left up Conzelman. Continue climbing Conzelman until you come to the intersection with McCullough. Look for the trailhead for the Coastal Trail and enjoy the ride down. At the bottom you have several trails to start your ride from. All fire roads, nothing technical, but the Marin headlands are full of natural beauty and historical wonder. You can do it on a road bike too and stick to the paved roads, but wouldn't you rather have the best of both worlds?

Road for sure! You can also go south down Skyline into Pacifica or any number of other routes towards Woodside, etc.