The 686 is an all steel revolver, and the extra weight will encourage you to leave it at home at times. Also keep in mind that the 686 is an entirely different, and larger framed revolver. You really can't compare a J frame to a K frame in any meaningful way with regard to size and/or weight.
I won't argue the potency between .357 and .38 Spec.+P, but I don't see any real defense advantage of .357 over .38+P.
So for my money, I went with the 642 Airweight and would recommend you consider it also.
A .357 in such a short barrel loses a lot of it's ooomph, so you're not giving anything up in horsepower, but you're gaining a lot in carryability.
Also, I don't know about the 640, but the 642 is available brand new without the IL which is something I feel much better about NOT having in a carry pistol.
Here's a relevant thread on DefensiveCarry, and if you explore the links and note the comments, I think the consensus is that .357 out of a short barrel is NOT a huge advantage, at least not an advantage that is worth carrying a heavier pistol and having more difficulty making fast, follow up shots.
http://www.defensivecarry.com/forum/defensive-carry-guns/112006-38-special-vs-357-mag-2-barrel.html