Hi Bob,
Can't speak to Cotton Carrier, but I do use both a BlackRapid and a Spider holster. I tend to use the BR if I need to put the camera down frequently, such as getting in and out of a car. I find the cross body strap works great compared with my old Optech strap, both in keeping the camera from slipping off a shoulder and for quick use - and no strap to get in the way at eye level!
However, if I am walking about for a while I'll switch to the Spider holster. Kind of scary at first not having any strap, but in truth I've yet to have a body slip out of my fingers whatever the strap. Actually not quite so fast in a "quick draw" situation but - and this is the biggy for me - no weight on shoulder/neck, all on the waist. It also gives me an excuse to put a couple of accessory/lens pouches on the belt and not carry a backpack at all. I use Kata pouches.
I haven't found any particular problem with the Spider belt or leg guard, but then I'm no mountain goat either.
Whilst it might be considered extravagent to have both systems, compared with the cost of the camera gear itself I'm very happy to have both to hand.
Regards, Martin