i think touring bikes like atlantis of rivendell or long haul trucker of surly would be good both on and offroad. thereīre lots of people who have experiences with these bikes perhaps they can give you a more detailed idea of them. nevertheless, i still think a mountain bike would be a good choice because with it you can do both offroad and commuting and itīs got a pretty upright position. more importantly, itīs got a bigger stand over clearance which in my opinion is something to consider when commuting.
but no matter what kind of bikes youīre going to buy make sure you get a good fit. take a look at these articles, they might be of some help.
http://www.coloradocyclist.com/bikefit/,
http://www.rivendellbicycles.com/htm...framesize.html
even though i mentioned atlantis i donīt think itīs a good idea to go after it because itīs really expensive and you probably can get something cheaper that does the job as well.