If barrel length, chambering and grip size are equal, then the question boils down to two criteria.
#1: Will you shoot a whole lot (100+) rounds of full-house 357s per week, every week, for five years or more?
If yes, buy the M686. If no, proceed to #2.
#2: How is the balance of either gun to you?
How's it point, how's the steadiness,
in general: how good does it feel in your hand?
If the M66 is best, pick it.
If the M686 is just as good then buy that one.
If you've got the cash, and it's okay with the wife, buy both! They're both awesome!
$.02, -Kframe