The major difference between the K and L frame is the diameter of the cylinder. On the K frame the cylinder is 1.45 inch in diameter, on the L frames it's 1.56 inch in diameter. Using a slighly larger diameter cylinder on the L frame allows the barrel to be mounted a bit farther from the axis of the cylinder. By doing this the flat to clear the gas ring on the K frame was eliminated. A secondary benefit is that it allows 7 rounds of 38/357 instead of 6. Now, one very minor quibble point is that this will result an a slightly higher bore axis, however since it's only 0.055 inch I don't think that it's enough that anyone would notice.
As for the grip to trigger relationship, they are identical between K and L frames, so you really won't notice and difference in size or reach to the trigger. As noted, the round butts permit the use of grips that I would consider just tiny, so shop around for a round butt early L frame or just purchase one of the newer guns which all use the round butt grip frame.
As for balance, I am NOT a fan of full underlugs. I think they are ugly and they do cause a muzzle heavy balance. It's one reason why I chose the 620 for my L frame 357, it came in a half lug profile that I consider much better looking.
The other reason is that the tensioned barrel used on the 620 results in what I believe may be the most accurate S&W ever produced. BTW, I've shot my 620 with a scope and am certain that it's capable of 1 inch groups at 100 yards, however I'm not a good enough shooter to do that, best I could do was 7/8 inch at 50 yards.
Now, if you aren't that interested in pinpoint accuracy, Lipsey's is currently listing a 686 Mountain Gun. It's a half lug profile with an old style tapered barrel and I would expect that it will balance as ideally as the older tapered barrel K frames such as the model 15 or 67. It's also listed at 32 ounces, which is darned near exactly what the 4 inch model 19 weighed in at. As for it's accuracy, I would expect it to be capable of 6-8 inches at 100 yards, which for a handgun really isn't too bad. Fact is if I could afford one right now, I'd have one on order.
Frankly, if S&W had the good sense the Lord gave a goose, they'd just add a dash number to the model 19 series and produce a model 19 on the L frame with that tapered barrel. They could also do the same in stainless and call in the model 66. Aint no Law that I know of that forbids using a change number to signify a shift to the improved L frame. IMO the L frame is in reality nothing more than an improved K frame, both feel nearly identical in terms of size and the only real failure in the L frames is that full underlug they tacked on almost every L frame offered.
