If you're looking to commute and bike for fitness, I would almost reccomend a mountain/hybrid bike. Each type definately has its pros and cons, the pros of the mtn bike being
*the strength of frame/fork/components
*width of the tires.
In my mind, it makes mountain bikes more adaptable.
They also tend to be a little cheaper (Specialized Hardrocks and Rockhoppers going for ~$450
However, they're less comfortable on long (and I mean long) rides, but should be alright for a commute of ~10 miles.
They're also not as fast and require more effort to move.

Road bikes, on the other hand (these are my perceptions... I don't actually own one)
*More expensive for a quality bike (generally, though you can find nice deals)
*Thinner tires may lead to less stability (though cross bikes can be equipped for wider tires, and tires with decent tread, so this is negligable)
*Sleeker, faster, lighter (generally...)

My bike was purchased to be an all-around-user. I couldn't fathom the idea of a bike that had to avoid pot holes or bumps/curbs/etc... and I wanted to be able to take on trails when I was capable of it. It's a '03 rockhopper hardtail, now with rack and trunk, and smooth tires, too. I can't *quite* keep up with roadies who are really cruising, but I do alright.