In reply to JessClmbr: Due to yesterday's miniature Siberian winter when I came to cycle in, I took the mountain bike (a Kona hardtail), and was thankful for it when sploshing through slush.
When it came to return home and things had mostly dried out, I discovered just what a difference the roadie makes. The mtb felt sluggish and uncomfortable for the return journey, and took me considerably longer than my commute usually does.
It rather made me think, apocalyptic road conditions excepted, I'd rather not use the mtb away from proper off-road terrain, and if I had to go for a single bike for most purposes it wouldn't be a mtb - I like it for occasional singletrack blasts, but elsewhere it just feels a chore.
Personally if I had to have just one bike I'd probably go for something like a Genesis CDF, and get a cheap second set of wheels with slicks on for any entirely road based excursions