Heard great things about the Wimberley II, but I think I'll go with the RRS full gimbal. I like the fact that it can also be used as a nodal point pano head, and RRS stuff is top notch.
I had a Wimberly for my 600mm f/4 and it was wonderful. However, it would be overkill for the much lighter 500mm lens. I'd go with Neuro's recommendation, or use it handheld or with a monopod.