I have K70's on my 550 and love them..They are new, so I don't know about the mileage yet. Haven't had them in the rain yet, but I'm sure they will be fine as they were fine 40 years ago.
I have 404's on my 750, and they are fine also. About 6,000 on the rear and maybe 10,000 on the front before it's time to replace.
If I could, I'd put k70's on my 750, but it's a K8, so that's out for the rear. I would prefer to match the original inch profile tires rather than metric if possible.
It's pretty much a personal choice.