I've seen some gripes about the newer versions with the lock on them. Is there a mechanical reason I should perhaps look at older pre-lock versions, or is the aversion more due to, IMHO, the silly nature of the lock on guns?
By and large, people who dislike the lock feel the way they do on principle rather than practical reasons. There have been a very, very few documented cases in which the lock has engaged during recoil and tied up the revolver when the owner didn't want it to, but the number of verifiable cases is small enough in number to be statistically irrelevant.
The vast majority of people who bash the lock do so because they are staunch traditionalists who just don't like the idea of change, dislike the lock for political reasons (they feel that S&W caved in to anti-gun politicians like Bill Clinton by incorporating the feature), or because they think that the lock detracts too much from the look of the gun. Unfortunately, many of the people who dislike the lock for these reasons attempt to blow the small number of documented problems completely out of proportion in order to justify their own preferences.
Unfortunately, many of the lock bashers also don't seem to be able to remain mature when they discuss the matter. Some of the more irritating lock-bashers don't seem to be able to refrain from a tirade about it any time a new S&W product is mentioned even if the discussion isn't about the lock. Likewise, many lock bashers also have stooped to the use of childish invectives like "Smith & Clinton," "Safety Wesson," or referring to the lock as the "Hillary Hole." When you see someone engage in these types of behaviors, you can pretty much be sure that they have an axe to grind and that any claims they make should be looked upon with a healthy dose of skepticism.
As to the original question, the 686 is built on the S&W L-Frame which is the larger of their two medium-sized frames (the next smallest is the K-Frame) while the M27 is built on the N-Frame which is their second large frame originally designed for bigbore cartridges like .44 Special, .45 ACP, .44-40, and .45 Long Colt.
The L-Frame was specifically designed for the .357 Magnum cartridge in order to remedy some of the issues that were sometimes encountered when copious amounts of full-power .357 Magnum ammunition with light bullets (less than 140 gr) were fired through K-Frame revolvers including the Models 13, 19, 65, and 66. What S&W basically did was keep the gripframe dimensions of the K-Frame the same while enlarging the frame window enough to allow a thicker forcing cone to clear the yoke. While the first L-Frames were all six-shot guns, S&W eventually figured out that the larger diameter cylinder allowed by the bigger frame window could safely contain seven shots and the 686 has been available in both six or seven shot variants for several years now.
The N-Frame, on the other hand, was more than big and beefy enough for the .357 Magnum cartridge right from the start and it's doubtful that you'll ever see an N-Frame .357 Magnum with a cracked forcing cone (they're very thick). The N-Frame does, however, have a larger gripframe and longer trigger reach and may not fit some people with smaller hands as well as a K or L-Frame revolver would. The N-Frame has an even larger frame window than the L-Frame does and while most have six-shot cylinders, the 627 and some Lew Horton limited run M27's can be found with eight-shot cylinders. The N-Frame .357's have shorter cylinders than the K and L-Frames do so while they are perfectly capable of handling just as high, if not higher, pressures, they cannot use certain long-nosed bullets like the 173gr Kieth bullet in .357 Magnum cases (though many people handload these bullets to magnum pressure in .38 Special cases for use in N-Frames with no ill-effect). While the N-Frame is probably one of the most durable .357 Magnum revolvers ever made for SA or relatively slow DA shooting, the larger, heavier cylinder does put a bit more stress on the hand, cylinder stop, and cylinder stop notches making the N-Frame a bit more prone to timing and lockup issues than the L or K-Frames if lots of hard, fast DA shooting is done though the difference will probably be negligable for most shooters.
The biggest tangible difference between a 686 and M27 for most people will be the balance of the gun. While both revovlers are very similar in weight for a given barrel length, the distribution of that weight is very different. The 686 uses a heavy, full-underlug barrel which places the balance point further in front of the shooter's hand particularly with a longer barrel while the M27 uses a tapered barrel with a half-underlug that places the balance point further back and nearer to the shooter's hand. Which one is better is really a matter of personal preference but shooters such as myself who are more accustomed to older S&W revolvers tend to prefer the balance of the M27 while shooters more accustomed to newer revolvers or those with full underlugs like the Colt Python or full-lug Ruger GP100 tend to prefer the 686. Both are excellent guns that will serve you well for decades and the only real way to determine which fits you better is to handle, and if possible shoot, both.
