I have tried reading both GRE and GMAT RC.
I found GRE RC slightly a bit harder than GMAT RC. Why?
1. GRE passages are more history and scientific based,
e.g. the theorem given by this "great philosopher" in 1880s, or the "findings of this new homones" debate. On the other hand, GMAT RC comprises of everything - from business case studies to current development in the world to passages found in GRE RC.
Personally, I prefer GMAT RC as there are business world passages in which I can relate to.
2. Correct me if I am wrong, GMAT RC has around 400+ words as compared to GRE RC of 500+ words. 100+ words can really stretch your brain cells.
So my personal comments for people who are not strong in RC is GRE RC can help to train someone like me who aim for the sky in order to hit for the trees.