If you are racing, or thinking about it, get a light, efficient bike, with a double chainring up front and closer spaced gears. This allows you to choose the perfect gear for going fast, be it training or racing. When you ride on the road, you tend to sit in the saddle for several hours. If your seat is rubbing you the wrong way, big trouble is heading your way. Extreme Sports will work with you to find a seat that fits your anatomy.
Road bikes tend to be more expensive than mountain bikes.
STI, described above, adds a considerable cost to the price of a road bike. Also road bikes tend to be lighter, and need to be engineered and manufactured very carefully to make sure that they are strong enough. Ask yourself where you are heading with your bike. If you want to ride trails and do some tricks, look for a dirt jumper. Ramp and freestyle tricks need to head in the freestyle category.