I've had my m645AF around my neck hiking all day with a single 35 or 45mm lens, no problems.
Tried the same with the Kiev88, and no go. The 67 is even bigger and heavier than that.
The worst thing about the 645 is actually taking portraits, there's a recipe for wrist strain right there. The rotating back on the 67 has its advantages there.
For travelling and hiking, I think 645 is a no brainer, unless you want more exercise.
Even smaller and lighter again are the folding and rangefinder 67 and 69 cameras like Fujis, but they're a lot more expensive (probably because they're more popular with people who want the portability).
The lenses for both are great, and most MF is cheap as all hell these days. 645 chews less film too, and unless you want to print 16x20 and up you probably won't see the difference...