Upload & Sell: Off
I have used the Wimberley II and Mongoose 3.5a on a 500 and 600.
The Wimberley is easier to mount both on because of the sidemount of the Mongoose but I like everything else about the Mongoose. So much that I sold my Wimberley and use the Mongoose exclusively.
The Mongoose pans nicely, is easy to balance, and has a nice drag. The lens and camera combo stays where you put it without having to lock it down. It has a quick release system for easy removal of your camera and lens. This is a very nice element when you want to remove your rig quickly from your tripod for some HH shots.
It does require the addition of a low mount foot for your lens for maximum performance. The low mount foot will put the center of your lens over the center of the tripod for better balance and smoother panning.
The big plus for me is the lighter weight because I trek with my rig. If your a stationary shooter with a heavy rig the Wimberley may be a better choice.