Ok, had a play with both over the weekend and Amazon will be getting one back.
the one I will keep is the small rig strap for these reasons as they apply to me personally. Others might not be in agreement though.
1. The strap is longer than the slide so can be made longer to suit individual needs, I liked it as it could be more adjustable length wise and did not let the camera just knock against my hip bone.
2. The shoulder pad on the small rig makes it so much more comfortable than the slide strap which is one width all the way around. It has a soft foam that sits nicely on your shoulder and is shaped to suit your neck. It can also move up or down the actual strap to get it more comfortable. The slide does not have one I did notice it around my neck but not as much if hanging over the shoulder.
Also I did not like the grippy bits on the slide but you can turn the strap over and they sit on the outside.
3. The quick release tags are slightly smaller but are really strong and have a ball bearing in them which I presume gives extra locking in, but I don't know for sure.
They go into the strap connector very similar to the slide ones, in fact they are more or less identical apart from they do not fit each other.
4. £25 cheaper as well, since I'm a skinflint that helped a lot.
What I did like about the slide was the buckles which have a metal arm to free the strap for adjustment. The small rig has the same adjustability but it is not as quick. Having said that I don't know how often I would adjust the strap if at all when I have it set to suit me. The slide box is more fancy, colour printing of setup and a fancy bag if that is your thing. Both straps come with extra tags and a plate too attached to tripod mount on base of camera. The plate for the small rig has four anchor points whereas the leash only has two (well mine did)
Hope this helps anyone that is thinking about the small rig strap, for me it is a winner and will be replacing my Optech Pro loop strap.