The RoninMX is a great gimbal.. and Gremsey is pretty much identical in functionality. The main benefits of Gremsy is 1) much lower weight, 2) newer design of payload carrier, 3) Intel-Hub interface, 4) Mount Head, 5) smaller height unit.
The RoninMX is EOL and it's Used price is a lower cost, which may appeal to many. Balancing the payload camera, the Gremsy is always faster in my opinion. The RoninMX rail adjustment seem more touchy to obtain a balance in all axis.
Mounting in either M600Pro or S1000... as mentioned eariler, the mount dimensions are different... something you can correct in S1000 to use M600Pro rails for Zenmuse payloads and others based on this mount.
But for RoninMX or Gremsy Gimbals, you don't need the rails... you can get a 3rd Party "mount puck" that accepts the gimbal pivot head directly. These are nice because you normally don't have to remove them to mount rails for other projects.
Another point, maybe it's been the 2 RoninMX that I have... but when you turn or yaw the platform... I've noticed the RoninMX may hold a horizontal tilt for awhile. The Gremsy's tend to be more stable, maybe due to newer technologies.