They claim the torque converter and powershift only has an advantage right at stall. I dunno??? was pushing up material one day at a sand pit with a D6N and a 750j side by side. they were pretty comparable in the cut but when we started to climb the 750 seemed to lose a little steam.
I don't have enough experience on dozers to be certain, but hydro machines seem to have a slight advantage at medium speed earthmoving, but seem to act up and bog down in very slow heavy pushing. I'd imagine though that ripping rock would be hard on the pumps and hydraulic motors because of all the jerks and stop/starts as the rock cracks and gets ripped. I'd expect it to cause premature wear and failure of the hydraulic pumps and moors. For a finish dozing, I'd think hydrostatic drive would be fine because it is generally smooth and you normally aren't pushing the dozer to its stall limits. Once again, I am not a dozer operator, and so these are just my hypotheses and I may be wrong.
The D6M that I tried for a bit seemed fine on slopes, but I can imagine that a low drive tractor would have a somewhat lower centre of gravity.
Get whatever you want tommy1984, but I can't see why production won't matter unless the job is too small to justify buying a recent dozer in the first place. Also, I wouldn't dismiss the D6R for its steering, I'd suggest you give it another try and see if you can get used to it. I do know for a fact that every bit of force is very important when land clearing and breaking rock, so keep that in mind. Still, if your slopes are so steep that a high drive machine may tip, then I guess it may be safer to go with a low drive unit.
P.S. The $50k price sounds oddly low for a D6R or a 750J. Be extra thorough when inspecting the machines.