If I were in your scenario, I'd want a second bike to have as a spare, and because having a non-derailleur bike in the winter greatly decreases the amount of maintenance required. You could go with a SS MTB, perfect for winter and MTB, and your other bike will stay in better condition.

I imagine that with a bar swap though, I could be pretty happy with my Giant ATX 780 as an only bike. As long as it isn't a race, I don't have much issue keeping up on a road ride, and while I haven't tried it yet, I wouldn't be scared to do a bit of off-road on it either.