I bought two 646s earlier this summer, one to shoot and one to sell.
I have shot two IDPA matches with one and recently sold the other one.
In a recent conversation with Steve of Ranch Products he told me that he hoped to have his first run of moonclips for the 646 duriing the Thanksgiving wekend and that they would be $1 - $1.25, which is about the same cost of 610 moonclips.
I have had a 625 for many years and while I really like it I personally think that it can't compare to a 646 with blance and pointability.
I have never shot any real hot loads out of it so I can't comment there but I have shot some 125 grain sizzlers out of 686s and while you knew you had touched off a stout one it was never what I would call severe.
Perhaps you could get someone to let you try both before you buy or just buy both and sell the one you don't like. I think they both are sellinig very well now.
One thing also, they only made 900 of the 646s and have made LOTS of 610s.
JNT