I haven't used Fine Grain Release Positive, but I have shot CMS20 (the original version). CMS20 is repurposed microfilm stock, designed to produce a two-value image: clear film, or full Dmax black. Getting a good gray scale out of it requires exposing accurately and using a special low-contrast developer. In some ways, it's similar to Tech Pan. Fine Grain Release Positive is intended to provide a full range of gray tones, with approximately normal processing in cine developers (D96 and similar). With the correct development time, FGRP will produce a good range of gray tones in almost any common black and white developer, though like most films, some will produce more pleasing results than others. It's made very slow because light is readily available (it's easier to give lots of light than less, in the process of printing a film like P.30 or XX Negative to produce a positive for projection), and it needs to have the finest grain possible to avoid artifacting that can make a negative that isn't terribly grainy seem like sand and gravel on the screen (at 100:1 or larger magnification).
So, completely different original design criteria: highest possible resolution at contrast that favors text and line drawings (and, due to resolution, tolerates half-tone well) vs. gray scale that matches or exceeds that of the negative film used in camera, with fine enough grain not to produce artifacts and tractable in common cine developers. Both can produce excellent, even stunning results -- but comparing them is comparing apples and aubergines.
Spotmatic SP, Super Takumar 50/1.4, Adox CMS20 (original), EI 20, Caffenol LC+C, scan from 8x10 print