Bike to Work: Good for the Heart, Good for the Wallet

I posted a link last week of ways of saving gas. Celeste and I are certainly conscious of the rising price of gas, especially since we live in one of the states with the highest price of gasoline (the closet gas station to us is selling regular unleaded at $4.73!). One thing that we're trying to do is ride our bikes as often as time will permit. Now that we've moved to Mountain View, my commute to work is less than three miles. So, I've been trying to ride my bike to work about four times per week. It's a great way to start the day -- gets the heart pumping and the mind thinking. At first, I was lugging all of my stuff on my back (not too comfortable when you've got a laptop in the bag). However, we recently went to the REI used gear sale and I got a rack for the back of the bike. It's funny because growing up, I used to think that kind of stuff was for geeks. Now I think it's kind of cool. :) Of course, I may have just turned into a geek. ;)

The bike I've been riding to work is actually Celeste's bike (I've claimed as my own in the paragraph above) . I got a bike a few months back for $25 which is too small for me (we mostly just got the bike to see whether I'd enjoy riding, which I've learned that I like tremendously). I've since learned a little about bikes including sizing, etc. So now I realize that I need a new bike, but I'm not sure whether I should opt for a mountain bike or a road bike. I think it is fun to go off-road, but my main purpose for the bike is commuting to and from work. In a perfect world, I'd get two bikes (a road and a mountain bike). Any suggestions?