UPstrap comes with wide black rubber bands that cover the metal loops. As mentioned on my previously entry, I use them on my D3x and D3, and a recently purchased a smaller one for my P6000. I have had no problems with scratching or damaging my camera bodies with the metals loops. The rubber bands are very secure around them. The rubber bands (2 on each side of the strap) also work well to secure the loose ends after adjusting the strap. I haven't had any incidents with the metal loops affecting the cosmetics of my camera bodies...as of yet.
I think the OP/TECHs are great straps as well, so you should be happy with either. Their grip is not as strong as the UPstrap, but far better than most of the other straps out their. With larger, heavier cameras, especially with heavier f2.8 zooms attached, the issue is having a strap with the best grip to prevent slipping when I'm carrying one of the cameras hanging off my shoulder. A D300 is much lighter and the OP/TECH strap would probably work just as well.
I'm glad to see you taking your time to research this issue. With the investment made in bodies and lenses, it amazing to me how many people don't even consider the one piece of equipment that can risk their investment...the strap!