I've commuted on both and for riding on city streets I much prefer the road bike. The mtb was just too heavy and slow, even with slicks. I can get around much quicker on my road bike so that is what I commute on. I like the quick acceleration and nimble handling of the road bike. My commute is just under 10 miles one way and I've been doing this once or twice a week since January (twice a week for the past few months). I've had one flat on my ride home so far and it was a quick easy tube change and I was on my way.
The mtb was a late 90s Trek hardtail. It was a decent bike (I paid about $600 for it new back in the 1997 or 1998, it was originally over $700). I think the model number was 730.
My road bike is a 2000 LeMond Zurich (steel) with Campagnolo Chorus 9 spd group. I have Look Carbon KEO pedals on it and a Polar CS100 with HR and cadence (I also use this bike for my weekend rides). Tires are 700x23 race tires on Open Pro/Record wheels. This bike can easily handle my weight plus as much as I can handle on my back (which isn't much...maybe 10 lbs). I never jump curbs though because you aren't supposed to ride your bike on the sidewalk and there are no curbs in the street.
My ride is all up and down. Round trip, my commute is just under 20 miles and over 1500' of climbing. Gearing is 53/39 front and 13-26 rear. I hit speeds well over 30mph at a few points on my ride. Average speed is generally 16-17mph. I know I'd be much slower than that on a mtb. I'm in North County San Diego near the coast FWIW.
I frequently pass guys commuting on mtb's if that means anything to you.
