I have a 4" 586 (blued 686) and it feels perfect in my hand. The 620 felt a bit light and the 6" 686 full lug felt slightly muzzle-heavy. Both were barely noticeable difference, and if I only intended to use the gun for home defense the 620 would have been fine, and if the 6" was only for range, it would have been fine. The 4" full lug was a perfect combo of both worlds... FOR ME. Please, I urge you, go with what feels right in your hand. And shoot all the guns if you get a chance.
Also, my 586 was a limited run 586-7 from last year. Yep, 7 shots. I know "a revolver is only supposed to be six shots," but who says? And why don't semis have a stupid rule like that? Anyway, on a seven-shot, the cylinder notches are between the chambers instead of above them. Theorhetically, this thicker chamber wall should make for a stronger chamber. Regardless, I've put my 586 thru the paces with all kinds of hot loads and to me, the only disadvantage of 7 shots is that it isn't 8... although Smith makes an 8-shot .357 as well.
My 586 is my favorite gun I have ever shot. .38s are like plinking soft .22s out of it and it can throw fireballs downrange with big loads and handle them with ease. It is also scary accurate. I mean, really, I am not as good of a shooter as this gun makes me look. 586, 686 or 620, you are not going to be disappointed... and you'll probably end up buying another one in a few months, anyway.
586-7, limited edition, 4" .357 magnum
Same gun firing 110grain SJHP Magtech .357 magnum two days ago.