I think the optional 6mm provides less resistance. Even at 6mm they are stout and will have a very very long life span.
My YZ4 just arrived going to the post office to pick it up. R&D Japan $406 shipped to the States in 3 days. Big thanks to my Bro for hooking me up with R&D, as well as a big thanks to Tim at R&D for getting it shipped out lightening fast.