I commute Long Island south shore to central Brooklyn, usually on a Miyata touring bike. The tourer has fenders and durable tires - Avocet Cross K's, inverted tread with Kevlar, and are pretty bombproof, if heavy. The Miyata has a rack for panniers, which I prefer over a messenger bag or backpack, especially on hot days. I only recently removed the fenders as I found I wasn't riding in the rain. This is the slowest bike

If I plan ahead and leave clothes at work, I use my Heron steel road bike - http://www.heronbicycles.com/, which accepts 27mm Ruffy Tuffy tires made by Rivendell - http://www.rivbike.com/
These tires are also very durable, but as the bike doesn't have a rack installed, I use one of Rivendells large saddle bags (Baggins Bag) to carry extra small items. Rding the Heron cuts about 5-10 minutes off my 27 mile commute.

I would not use a mt. bike, wanting a drop bar for comfort. The larger/heavier tires at 100psi make for a comfortable, yet reasonably efficiant ride, especially on windy days (along the ocean).

SB