I went with Sony Magnescales because I bought into the hype about them being impervious to dirt and coolant. I've never used any glass scales to know if this is a serious problem with those, but I think they are quite a bit cheaper.
However, the Magnescale's read head is still a fairly close fit to the encoded magnetic wire that runs the length of the scale. I did find that if coolant found its way in, that over time, it would build up and jam up the scale. Imagine my surprise one day when I was moving the cross slide and the encoded wire pops out the end of the scale. Yeoww! I did repair it satisfactorily but I did take more pains after that to deflect coolant from pouring onto the scale.
What else. There is a scale built by Newall that uses tubes filled with precision steel balls as a scale. These might be worth looking at.
However, since I have gone CNC, I haven't been greatly interested in plain DRO's. But, I would be researching the features that the modern units have that serve as sort of a CADCAM machinist's helper, rather than just a bare nuts position sensor, which is all I have.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)