You are paying extra for THX certification, a bit more grunt, and an extra HDMI zone by the looks. I have the RZ710 and am pretty happy with it. Depends on whether more power and THX is worth the extra for you (The answer is more power, the question is irrelevant isn't it?  ;-p )

The new 686 has THX certification... maybe wait for that?

Or, look at what THX cert. actually gives you sonically - and make your call from there.

Back 'in the day', it may have been a consideration, but today, where it's darned difficult to buy a 'bad amp' it is largely viewed as a marketing stunt to get people to buy one amp over another.