With lens feet from Kirk, RRS, 4th Gen Design and others which one did you go for for your EF 600mm f/4L IS II USM and why?
The RRS foot, for better balance on a gimbal while retaining handle function. I eliminated the Wimberley replacement foot as it can't be used as a handle. I'd never heard of 4th Gen Design, but looking them up, I learned that's the maker of the Mongoose side gimbal, of which I've certainly heard. Although the pic is tiny, it looks much like the Wimberley 'foot' - too flat to be used as a handle. I believe that is a consequence of the Mongoose and Wimberley side gimbal designs. RRS doesn't have that issue, since their side mount gimbal (which I have) can be adjusted horizontally to center the lens, as opposed to the one-piece design of the others.
I'm not sure what Canon was thinking with the tripod collar on the 600 II, and the 500 II seems even worse. Basically, the Canon foot is too far forward. It's fine with just the body, but when you put a 1-series or gripped body with a 2x TC, the center of mass is right under or very slightly behind the mounting screws on the tripod collar. The problem is the Canon foot sweeps forward, and a Wimberley P-50 doesn't sit back far enough. The Kirk foot is designed just like the Canon foot, with the forward sweep. The RRS foot can be used as a handle, and actually extends slightly behind the mounting screws on the collar. With the 2xIII and 1D X, the back edge of the foot is flush with the back of the 80mm RRS clamp on the gimbal at the balance point. So, with the Kirk foot or a Wimberley P-50 plate on the Canon foot, the back edge of the foot/plate would be inside the clamp - probably still plenty stable, but I'd prefer to use the full extent of the clamping surface. Alternatively, with it flush the intrinsic tension/resistance of the head would probably compensate for it being slightly off-balance - but I'd prefer it to be properly balanced.
Probably more detail than you wanted...