If I was going to have just ONE bike (perish the thought) it would probably be a mountain bike. Why?

The "economics of volume" enable companies such as Trek to sell $400 mountain bikes that are better built, and more reliable than a $600 road bike. A mountain bike makes a tough, reliable urban commuter just by putting on light weight slick tires and light plastic fenders. Urban potholes that would destroy the wheels on a road bike present "no worries" to the beefy rims of a "name brand" mountain bike. The MTB is a bike that can take a beating, week after week, yey not spend a lot of time in the shop.

On week-ends, an MTB becomes a "go anywhere" bike that can handle dirt trails, by putting on fat, knobby tires. So, it can take you places that you could never go on a road bike.

Security? Mountain bikes in the $300 to $500 price range "clutter" the urban landscape, so they don't draw the eye of the "better" class of crooks...the ones that know how to open even the best locks. Road bikes have a "high end" image, in part because even "entry" level road bikes cost more than $500...a crook can get a good price for a newish road bike.

You need to budget around $600 for your first bike with gear. About $400 to $450 for the bike. You also need a Kryptonite or OnGuard U-lock, a cable lock, a good helmet, a pump, spare tubes, and a basic tool set. With reasonable care, you can get five years or more of hard daily use out of such a bike...about twenty-five cents a day for transportation, better health, and fun and relaxation. Such a deal!

Stay away from "new" bikes that sell for under $200. They are NOT a bargain...most won't last a year in hard daily urban riding.