As far as the "in-between-lengths" go, I have a model 629-3 and find that the N-frame with the 5" barrel and full underlug balances very well, i.e. it doesn't feel barrel heavy and that balance makes it seem smaller than it really is. I don't own a 686, but have held many 3" model 686s and feel they have a similarly superb balance, whereas the 4" already feels barrel heavy in the L-frame.

You didn't mention what caliber or frame size you were interested in, but I think that in any caliber, a 3" barrel excels in concealed carry at the expense of velocity, while a 5" barrel shoots like a longer barrel without the awkwardness of the longer gun, but at the expense of ease-of-carry in most situations.