I have had both road bikes and mountain bikes, I prefer a mountain bike usually. If I knew it would always been pavement, I might be tempted on a road bike. Honestly, speed isn't a big thing to me as long as I am getting a good workout. I have a Cannodale mountain bike and when I have gone on tours, etc I tend to be a bit faster than most people anyway, so the additional control and options of a MTB I like. It is a hardtail, which is much lighter though.

I used to commute with a MTB too, but the road wasn't very good, so I needed the ability to ride over anything.