I commute 12.3 miles each way to work almost everyday of the week here in Phoenix AZ. I have pressed my trek fuel 98 into duty for this, the ride used to have a few miles of offroad, but that has since been paved. Accept for road construction, and bike path debri, I can ride my road bike, (cadex carbon fiber ultegra equipped) but using either of these bikes has some drawbacks:
For example, presently, my mountain bike has semi road tires, with flat resistant tubes, tire liners and tons of slime. The thing now rolls like a stuck pig, but it doesn't get flats. Also, the full suspension adds needless weight, as i keep the back locked out now that the remaing section of road has been paved. When i go mountain biking, I just hate my tires... Plus, I am too lazy to get up early and take the rack off the back, and nothing looks sexier than a rack on a nearly $3000 mountain bike.
If I choose to commute on my roadbike, I will need to upgrade the tires to something more flat resistant; plus, I have to actually pay attention to what i ride over, and cant jump the curbs so easily. Also, I hate walking in my road cleats; the mountain bike shoes and cleats are much easier to walk in. Once again, when i wake up late for a group ride, and go with the rack on; i will certainly be styling on my sleek ride...
So, I am kinda debating between: 1. Trek su100, and change the tires to some at least 80 psi tires so it will roll better... or 2. trek 7.5fx. The 7.5fx looks cooler, I might change to some armadillo 30something tires to get a little more forgiveing ride, and has the gearing for me to settle a few scores with a pair of arrogant road riders who I occassionaly encounter on the ride home... who insist on dropping me on the downhills, where my lower mountain bike gearing makes it impossible for me to keep contact...
Anyways, I am looking forward to having a dedicated commuting bike; any advice will be appreciated.
The 7.5fx is the only bike I own. I ride it about 100 yards up my gravel rd in the morning, then all pavement til work. It takes rough roads/debris very well with the 32c tires. I jump down from the curb to the street after work every day. The heads-up geometry is nice too.
The only worry I'd have for you is the lack of hand positions. My commute is only 6 miles each way, but when I go on longer rides I find myself wrapping my fingers around the shifters, resting my palms on the bar ends, etc.
Otherwise, it's a pretty fast, pretty rugged commuting machine which I enjoy riding very much. And that's what I want from my bike, a balance of fast and rugged.
Buy another set of mtn bike pedals, some thorn resistant tubes and some specialized armadillo all conditions tires (or conti ultra gatorskins) and ride your road bike. I've taken my road bike through construction zones and it can still be ridden, just at a slower pace. I think you'll be happier in the end than you would be with a hybrid.
I am going into my second season on a 7700FX. I did a few upgrades to the stock bike, mostly to lighten it up a bit more. It is really fast for a flatbar, not as good as my roadie, but not by much. It is a whole lot more stable and forgiving though, and still is plenty responsive and nimble especially for slow speed manuevers around obstacles or in close quarters. The Race Lite Hardcase tires have held up really well for me, but still are plenty fast considering their construction.
I've got a Giant Cypress SX which is fairly similar to the old 7500fx. It's outfitted with fenders, rack, panniers, etc. All loaded for work, it probably weighs about 40 lbs. Even though I have a nice road bike which I don't use for commuting, comparatively I thought the Giant was pretty speedy. Then I bought a cyclocross bike this spring. It has no rack, fenders, etc. On the days I ride it (decent weather forecasted), I carry all of my stuff in my messenger bag. It is way, way faster, even with cyclocross tires on it. Only drawbacks to it are no rack & panniers, so I have to carry my load on my back, and no fenders, so if it gets wet, I get skunk butt.
Personally, I think it's best to have more than one bike for commuting, even if just for variety's sake. They all have their advantages in different conditions. Except for the FS MTB. Not much use for that in the city unless you are trying to get a 20 mile road bike workout in 5 miles. The 7.5fx (Jamis Coda line is better IMO) has the advantage of being able to load it up with rack/panniers/fenders (the latter probably not needed in PHX), so the weight is off your body. CX has the advantage of being as light as a road bike. With both, you can hop curbs and ride through dirt roads without white knuckles (with decent tires). Another advantage to the CX is that you can get an extra set of wheels/road tires, and it becomes a very capable road bike. Hard to call a flat bar bike a true road bike.
Personally, I wouldn't ride an expensive road bike to work. First, it might get stolen. Second, you'll wear it out during mundane riding. Save it for the fun rides. Not that it's not fun to ride to work (that's a big part of the reason for doing it), but it's like commuting in a Ferrari vs a Civic. I figure if I rode my nice road bike to work, I'd never feel the need/desire to take it out for a non-commute ride.
Ride the road bike for the time being. At the same time, methodically watch Craigslist for bikes of interest and be ready to buy. Advantage to buying used is if you don't end up liking your choice, you can probably sell it for the same as you bought it used. New you lose 25-50% almost immediately on resale. Or go out and buy new ones if you prefer. My only beef with off the shelf CX bikes is that they are mostly aluminum these days (exceptions Jamis Nova, Bianchi Volpe). Since you already have a CF road bike, you're going to find aluminum a harsh reality. I'd look around for a steel one, though it could be a bit of a wait. On the other hand, many people think steel is passe, so they are not quite as sought after as an aluminum CX is.
My Waterford CX was listed on Craigslist for a few days before I snapped it up. Lucky me. Then again, I've seen a brand new Ritchey Swiss Cross bike listed multiple times (still listed in Denver!).
Best of luck whatever route you choose. Bikes are cool!