A lot of people here will tell you its better to drive the quill then the knee. Even IJ made a quill drive after he did the knee drive. Some will say that driving the knee is better because of greater travel.
I did a conversion on a 10x50 sized knee mill with the knee as the z axis. This machine has been running this way for about 2 years now. So far I cannot see any excess wear the slidway or have noticed any increase in slop. The knee is counter balanced so the servo does not need a lot of grut. It runs at about 135ipm max but I plan to increase that up to 500ipm soon. As far as positioning goes I can hold about +- 0.0002 in repeatability all day long.
The reason I went to a driven knee was that I thought the knee would hold its rigidity better over the full length of travel. So far so good.