I purchased one of the knock off spider holsters earlier this year. Terrifyingly the first broke. The back of the holster belt is held together with a single plastic O ring which was quite flimsy. The newer ones are supposed to be stronger.
Anyway....regardless of whether you go with the original or the knockoff, one of the biggest issues I found after shooting with the Black Rapid's for a couple of years is speed. Switching between cameras on a BR strap is quick, easy. Grab the camera you want and go!
With the holster you have to line up the camera you have in your hands with the slot, insert it, let it come to rest. Next grab the other, twist it and pull it up. And if you have the safety lock on (which only the original spider let's you disable) then it's a lot slower.
Personally I went back to the BR strap since I found swapping meant I missed moments that I would have otherwise caught. That said I don't get back pain as a result of weddings. If I did then I may reconsider.