The system I fitted to my M250 uses Sino display (which I bought secondhand) and an optical scale fixed along the back of the bed using existing tapped holes. There is plenty of room at the back of the bed so the size of the scale is not important and optical ones are generally cheaper.
For the cross slide I used a magnetic scale because it only adds 10mm to the right hand edge of the casting which is pretty insignificant.
There is a lot to be said for a pick-and-mix DRO setup, most scales (magnetic, optical types) have similar outputs so can be selected to suit the location.
Ian P