You will probably be slightly faster on a road bike, but there are other considerations. I can't imagine commuting by bike in your climate on a bike without fenders. For that reason alone, a touring or cyclocross bike would be far more practical to commute on than a pure racing bike, which is what most road bikes sold now are.
I use an old 7-speed MTB with old components because I often leave the bike locked outside all day. I don't find I am much slower on this bike than my 'road' MTB.
Originally posted by booey
My mtb lacks suspension so that is good. I live in portland oregon so it rains all the time [which is partly why road bikes scare me, that and the fact that I always see people with road bikes injured] . Should I get faster tires or are knobbies better?
Knobbies will only give better grip in loose surfaces where they leave an imprint. If you are trying to avoid wiping out, you definitely need to avoid cornering in the wet on knobbies! On a commuter MTB - for both dry and wet tarmac - 1.5 'slicks' are about the best compromise (the slicker, the better), though larger slicks will provide somewhat more shock absorbtion and grip with slight penalties in rolling resistance and weight.