What type of bike to buy?

I'm planning on getting a new bike soon, since the weather is getting warmer, or perhaps summer is around the corner in 2 more months. I'm debating whether to purchase a mountain bike or a triathlon bike. I do bike alot, but not on mountain rodes, and I do want a fast, comfortable bike, but I don't know which type to get. Does anyone know the major differences betwen the two?

Tires do play a big role. Is it okay to ride a triathalon bike on regular roads suck as sidewalks, and paved rodes? Will it slip?
Also, what is the point of the suspension (front & under the seat) on mountain bikes?

First lets make sure your talking about the correct type of bike. I think your confused about what a triathlon bike is. A tri bike is designed for one thing and one thing only, to get from point A to point B as fast as possible. It is designed for aerodynamics and speed and not as concerned about weight as a true road bike would be. Triathlons are usually raced on long straight roads with very little hills and only a few turns here and there.

Here is a triathlon bike:

http://www.feltracing.com/store/images/l...

Here is a road bike:

http://www.feltracing.com/store/images/l...

Notice the big aerodynamic tubing on the tri bike, it is also equiped with aero bars which allow you to stretch out further and reduce wind drag. The aero bars though are very unstable to ride on in the city and on bike paths where you have a lot of quick stops and turns to avoid people and things. This type of bike is typically used for training rides on back country roads away from everyone because of the control required to ride aero bars or only at races where the environment is controlled.

If you are talking mountain bike vs triathlon bike there might be an alternative for you. Check out a city bike, this gives you a bit of both worlds. You get the more upright geometry of a mountain bike allowing you to see better where you are going. It will be more comfortable than a road bike and definitely more comfortable than a tri bike. The gearing and components on the bike though are typically equivilanet of a road bike with the thin road wheels and tires. This will allow you to maintain a good speed on the road while being comfortable.

Here is one of many city bikes

http://www.jamisbikes.com/usa/bikes/08_b...

Road bikes for speed, MTBs for terrain. since you prefer speed, a road bike would be better. but they are generally pricier than MTBs.