A good compromise would be the M66-8. I have had a number of K frames in 2.5 and 4 inch. The 4 inch is no more difficult to carry concealed. The M66-8 will handle the hot loads with no problem. And parts will be available.

I also had a M696 and a 4 inch M686 and both carried OK with a proper holster and belt. I used a 4 inch M581 for a duty revolver and carried it off duty too with no problem.

In your case I would keep both of them. Carry the M66 and practice with the M686. They are about the same in handling.