It depends on what it's for if you are concerned with utility. As was the original idea, the K frame magnum is a .38 that can handle a limited amount of magnums. The 686 is a heavy duty revolver that is lighter than a N frame such as the M27/28 and it can take lots of .357 without a problem. If you are going to carry it all day, I would stick with the M66. If it's mainly going to be a range gun or house gun, then the weight doesn't really matter and the heavier gun would be easier to handle with magnum ammo.
The origina reason for the K frame magnum was to lighten the load on a cop's belt and it is a good compromise between power and portablilty for that purpose. I have all three sized S&W .357's, and one in J frame too(640-1), and find that since I do most of my range work with my .38 reloads, the K frame is fine and the others about the same. For carry, I pick the M19/66 most often though. In my case, I have wound up regretting that I traded/sold the guns I have.