S&W made very, very few 681's with a lock. 97% of them or more don't have it. I've owned several Rugers; liked them. But they are not forged steel S&Ws and won't ever be batting in those leagues.
I have had a 681, and two 686s - one 2.5" and one 4". Of the three, only the 4" 686 is still in my house. And it gets pride of place in the home as it and a full-house Novak Custom Colt 1911 are the only two guns I keep as nightstand guns. Lots of others can do the job just as well, but those are the two that get the honors.
The 681 is fine, but the sights are just not as quick or accurate as those on a 686. And the S&W adjustable sights have proven plenty durable and rugged over time, no reason to worry about them on a carry, duty, or field gun.
As to the 2.5" 686, it's just too bulky to me for a cc gun, and for a light-use weapon like that (e.g, not a lot of rounds down range typically vs. amount carried), the 19/66 just shines for that.
The 4" L frame will "out shoot" a 4" 19/66 or even a 27. The L frame has more weight than the K, but the full lug makes it so much more softer shooting. the K is of course handier, so it depends which you value - quick pointing or softer shooting/faster on 2nd shot. Weight is the same between a 686 and a comparable N (3.5" or 4" 27 or 28), but the weight is lower and more compact so it handles a bit better and has better recoil characteristics.
For flat-out shooting, pick a 4" 686. For sexiness, a 3.5" 27. For carry, a 2.5" 19/66. I've got one of each and two of the latter. The 681 is a fine gun, and if you can get one at a good price, go for it. But if prices are narrow in difference, I prefer the 686 for the sights.
K, L, N:
This photo may be more illustrative. All that weight under the barrel makes it less handy than a 19/66, but it still enables it to be, as it quite literally was, a "Python Killer."
