I own mtn and road bikes, but my favorite is my 'cross bike. In northern Virginia, the winters can be wet and nasty, which is perfect for taking your 'cross bike out on nasty dirt roads, moderate singletrack, slippery asphalt, etc. My bike is great for commuting and fun for fun's sake. I've even taken it to the local BMX jumps to show those punks how to really ride those bumps. You might consider getting a suspension seatpost to take some of the jolt out of the trails.
Wisconsin's weather made me give up on my road bike. I sold it to some guy in CT (in the roadbikreview.com classifieds) and bought a cyclocross bike instead. There's something about drop bars and cantilever brakes that makes me hot. Couldn't be happier. 8-)
PS--My cyclocross bike makes my MTB feel like a big motorcycle.
I'm lucky enough to have an MTB and a 'cross bike. I switch inbetween the two during races, which is pretty handy in thick muddy races, and the other can be washed. I actually prefer my MTB to my cross bike, as it handles better. Any one have any other views?
My race MTB seems to be more fun when I hang 700c wheels in it. OK, the higher BB makes handling in slippery bits trickyer, but you gain a lot of straightline-speed.
My MTB frame is faster btw, over 400grams heavier than the TRC road frame, with comparable sizes, it seems much more responsive.
When fitted with the right tires, cornering with 700c wheels it almost equally fast as on the MTB. In forrest trails I think it's a faster option.