Well, the
X6HR is new technology, much easier to use, smaller, lighter (but not significant). They now both use the same
strap, so no difference there.
I was never a fan of the leather
strap for an athletic watch, doesn't make sense to me, and
Suunto still does not offer an alternative.
The
X6HR is a bit more $$ but much better as far as I'm concearned, but if you like a real big watch, the Metron is 1/3 bigger.
Hope that helps,