@Ian_Colwell ray_ground filter is very well described in this design document and in the re-write for ROS2: https://gitlab.com/AutowareAuto/AutowareAuto/blob/master/src/perception/filters/ray_ground_classifier/design/ray-ground-classifier-design.md.
For the ring_ground_filter lets ask @amc-nu.
We (at Apex) use the ray_ground filter and are very happy with it. I am not aware of any performance comparison to the ring_ground_filter.