I have got a report that these two types of rear deraileurs have the same mechanical advantage:
- Shimano Dyna-Sys (which are marketed only as “10-speed”)
- SRAM E.S.P (which are marketed only as “9-speed”
Both take 35mm of cable to go over the whole cassette. Picture shows SRAM X.9 and Shimano XT, but any other derailleurs of the mentioned types would be interchangeable.
This means that either one can be replaced with another (provided that it can handle your biggest cog). I have not verified this myself. Remember that “SRAM 10-speed” deraileurs have different mechanical advantage, as do “Shimano 9-speed” ones, and there are no other pairs which are not designed for each other but work.
Maybe not a really useful finding in practice, but certainly an interesting one.