Robot Damashii and Revoltech have key differences:
Revoltech is primarily PVC construction; Robot Damashii has a mix of ABS, POM, and some PVC. The former is more prone to warping.
Robot Damashii is a figure that can maintain neutral stances yet retain full articulation. Revoltech is primarily for dynamic posing and for most, neutral stances cannot be achieved. As in straight posture, et al. I guess the easiest way to boil it down is that Revoltech lends itself to more exaggerated proportions, poses and stances more than Robot Damashii. Robot Damashii is like a basic all around action figure for collectors, while Revoltech is all about dynamic posing.
Robot Damashii tend to be taller than their Revoltech counterparts.
Robot Damashii uses regular joints while Revoltech uses a combination of mono-shaft joints combined with revolver joints.
Hope this helps.