I think a lot of it depends on your actual commute. I have no clue what your is like (miles don't mean as much as conditions), so I can share only my own experience. I live just outside the city center, have one job a bit further out in the city and another in the suburbs.
Right now, I own only one bike. A hybrid. While I'm scoffed at equally by MTBer and roadies alike, for my situation (tiny city apartment, bad roads, little off-road riding) the compromises made by a hybrid exactly fit the compromises imposed by my living and riding situations.
If I lived and worked in the suburbs, I'd choose a road bike without a second thought. But on cratered, narrow city streets in dense traffic where obstacles can't always be avoided, plus the occassional need for curb jumping, or darting through an alley, a roadie is not the best choice for me.
I''ve never owned an MTB, but I ridden a couple with friends who have extras. They're great and tremendous fun in the circumstances they're designed for. The main reason I wouldn't select an MTB for commuting is the gearing. Those teeny, tiny chainrings really limit speed (even more, I think, than the squishy suspension and tires)--and in traffic, even city traffic, speed can be your friend.
Where my hybrid shines is that it's lighter and much faster than an MTB. That makes it easier to flick around in dense traffic, and once I get out to the suburbs, the bigger chainrings mean I can crank right along--which the cagers seem to appreciate as much as I do. On group rides with roadies (once they're done laughing at my bike) I keep up pretty well too, until I spin-out around 32.
It's also more rugged than a roadie. I just don't have to be concerned with road conditions or equipment failure--which is a nice feeling when you're riding past crackhouses after dark.
The fact that nobody wants a hybrid makes me feel a little safer when locking it up. All things being equal, an MTB or a roadie is a bigger target. The other big benefit is that they're cheap. Mine was less that either a decent MTB or an entry-level roadie. Even if, despite my best efforts and locks, it does get theived, I'm out less money.
The saddle and seatposts suck on every hybrid I've seen. I replaced mine as soon as possible. To help overcome the wind resistance of the upright position at speed, I added bar-ends (the curved-in type). Even with these mods, I've spent less that I would have other bikes. Which is fine for my daily commuter.
Yes, I'm getting rid of furniture so I can fit a second bike in my apartment. It will be a roadie. But I doubt it will replace my hybrid for back-and-forth to work.