If you can FIND a 3" K frame I'd say buy it as quick as you can. A 3" K frame is just about the sweetest handling revolver made. I now shoot IDPA matches with one and don't think I give up anything compared to the 4" K frame I used to use. You can shoot one of these all day with 38 Specials and suffer no discomfort.
Your friend's 686 is actually an L frame, which is a bit bigger and will be slightly heavier than a K frame, although the grip sizes are the same.
If there is such a thing as a 4" J frame in 38/357 (I haven't heard of one, but who knows . . . ) it will be MUCH less pleasant to shoot, and will hold one less round, than a K frame. I have fired ONE (1) 357 round out of a steel J frame, basically to say I did it, and will never do it again. Even shooting 38 Specials out of one isn't a lot of fun unless you use big cushy grips.
As for "home defense," be aware that if you shoot one of these things inside a house without ear plugs or muffs you WILL lose some of your hearing for at least a while. If you shoot a 357 round you will lose more of your hearing for longer. In either case the damage may or may not be permanent. Although I do have some 357 revolvers, all those that I keep ready for home defense are loaded with standard pressure (not +P) 38 Special ammo - I haven't found +P to give much extra velocity from short barrels, but it does kick more which makes recovery for follow up shots more difficult.
Whatever you decide to get, remember that the secret to successful use in dire circumstances is PRACTICE, PRACTICE and more PRACTICE.