In my opinion, the difference is which is a "shooting" gun, and which is a "comfort" gun. The J-frames are much smaller, more compact, much lighter in weight. The K-frames are much easier to shoot well, especially under stress.

I have used both for many years, and I won't feel unarmed with either one on me. If I know that I am going into dangerous circumstances I will choose the K-frame every time, and carry it as a back-up to my Remington 870, my M1 Carbine, a couple of 1911's, and anything else that I can bring along.

If I am running down to the corner store to pick up a quart of milk, I don't mind slipping my old Model 37 Airweight Chief Special under my shirt.

6 rounds or 5 rounds? This makes less difference to me than "first round in the 10-ring, second round in the 10-ring, etc, etc, etc."

For defensive use I suggest that you carry what you are comfortable and competent with. If that is a K-frame, and you want a 5-shot revolver, just leave one chamber empty (under the hammer, preferably).

Best regards.