I agree that, generally, a road bike is about a third faster than a MTB. My personal experience having ridden a century on a MTB with slicks and a Roadie later is that life was just way better on the road bike when riding on the road. Way better and faster. Then there is my commuter/tourer that is geared like my roadie but heavy like my MTB and I still manage to have fun on it and ride long distance without fear of being too light or too fast. It is wise to ask others about these things because there are so many variables and you have to be settled in your mind about what you really want. Best of luck to you.