they sling different directions. R-strap worn on left shoulder, camera sits near right hip. Y-strap worn on right shoulder, camera sits near left hip. For a right hander, the R-strap configuration is easier to reach for.
Y or R, their concept is the same, a movable attachment point to your camera body, the Y make use of the neck strap lug on your camera while the R use the tripod mount.
Y system using the neck strap lug, I feel that the concept works well for range finder camera or DSLR with short lens, DSLR with big or long lens will strain the lug and its not very comfy to have your DSLR with long lens and with the lens poking outwards side way
the R system is better using the tripod mount but when using big long lens the tripod mount be strain badly if u use the camera mount and if u use the lens coller mount the camera may not hang vertically, maybe side way or even camera down with lens facing upwards
i still prefer the conventional neck strap, over the neck and under 1 arm and i can pull the cam to the side or back, have it long enough can also use it like the R system
Think the best place to find parts for DIYing your own R or Y strap will be beach road army market as there got lots of straps and those metal bucker or hook
I think nikontkl has highlighted the main difference. I personally am using the R-strap now and its definitely quite useful when walking around though sometimes the strap can start shifting down. Also if you use a long lens with no tripod ring it can be a little un-balanced IMO. Still does make carrying the camera around more comfortable.