Even that is not enough to make a decision on. "Dirt" can be alot of different things. I ride my RT 7-10% in the dirt. I ride out through the hard pack dirt roads in Ocotillo Wells OHVA regularly. I would never think of taking it out in sand or mud or the big rocks...but hard pack fire roads or access roads you could drive a car on...no problem.
If your dirt is less than big rock, sand, mud or serious "OFF" road then a 12 is a great bike. Very skilled people do take them into rougher terrain, but most do not. If your idea of dirt is fire and access roads that your average pick up truck or maybe mild 4x4 would drive on then the 12 is great. If you are talking MX track, single track woods, rough and sandy desert then a 250/450 would be the tool for the job.
But your comment on "MOST" of your riding being longer distance hiway and from city to city riding then I believe the 12 would be the best BMW bike for the job....not saying the only bike, but as far as BMW offers, probably the best bike for the job. Capable on 10-20% dirt if it is not too crazy, and 80-90% sport and touring hiways...that just screams R1200GS or GSA if you are taller and want a little bit more dirt worthiness....but again they get tall and top heavy.
I hope that helps...DEMO one for a day and see what you think...seat of the pants dyno test.