even if it spends just half its time on the road , you should avoid knobblies as they won't last long at all. I had an AC10 on the rear of my bike and that lasted all of about 300-350 miles before the center was pretty much bald (yes it is more powerful then the DT but i can imagine they wont last all that long even on a 125 (especially if you are riding mostly on the road)
I would recommend you get some half 'n' halfs (trail type tyres) . When i get a chance i'll have a look whats on the DR- they are more the road type, rather then full knobblies.