Hex:
I'm going to put in my 2 cents here without consideration of us talking about a Ford vs. Chevrolet decision. If you're going to really shoot that shooter a lot, you might consider a criteria I apply for buying what I'm going to use (not look at or plink around with): Look first at what those that consistently win competitive shooting matches use and do. I apply that pretty much across the board - from smallbore to highpower to shotgun sports - to really get what's going to serve you best, look to those that depend on their equipment to take home the prize. I don't think that necessarily means you have to invest more; you just have to invest right.
In the more than four decades of PPC and those that use revolvers in practical shooting competitions disciplines, Smith and Wesson K and L frame revolvers (as I'm sure you know, the 686 is an L frame) have dominated the field. If you narrow that down to those that sought/seek to earn the Police Distinguished Badge and Service Gun championships (which requires a stock revolver with virtually no external modifications), S&W has dominated even more.
Why is this? I believe there are lots of reasons, but the main ones to me are:
1) Ease of tuning for double action shooting - K and L frame revolvers are fairly simple to smooth and lighten internally and still stay timed. There are lots of armorers out there who can do that in short order at a low cost. I have a bunch of them that when people pull the trigger, they say, "Wow! How did you get that so smooth?" I would venture to say that almost all 6" S&W factory barrels on guns shot from a Ransom Rest with match wadcutters or ball will stay within the X-ring on a B-27 target at 50 yards. It does a great deal of good to one's confidence when you know that the gun will do its job if you do yours on sight alignment and trigger control.
2) Lower entry and maintenance cost - the price on a new 686 is comparable to a King Cobra but the Python, as noted by others, is way out there. I have been out of competition for awhile, but I think I'm right in saying you won't see many of those on the firing line. IMHO it takes a lot more smithing to make any of the Colts meet the standards for double-action shooting that will satisfy winners. That equates to more expertise on the part of the smith which equates to more money for the shooter. Also, when internal parts on K and L frame S&Ws need replacing (after a few thousand rounds), you're not going to break the bank or have to look very far to get them. It's also easier to find after purchase accessories at reasonable prices for S&Ws vs. Colts. I'm talking about speed loaders, grips, holsters, etc.
3) In the competition disciplines mentioned above, most competitors build their open competition guns (what some call a race gun in practical shooting disciplines) on K or L frame actions. In addition to all the points in #1 and #2 above, if you shoot an open gun and service gun with the same grip size, your consistently gripping the same way all the time. BTW, you probably know that the K & L frames have the same grip size and grips that will fit on one will fit on the other.
Now, after all that, let me say that I think the Colt Python is the best engineered and finished double-action revolver that ever hit the market. I'm not too familiar with the King Cobra, but I thought the old Cobra was substandard compared to Smiths. I would love to have owned a Python, but that would have messed up both my pocket (purchase and tuning) and my mind with that cylinder and sights going the "wrong way" and having to have a whole new set of speedloaders. Also, there was only one guy I ever met that I would have trusted to work on my Python if I had one. That's Lt. Reeve Jungkind, Texas DPS. Clearly the smoothest, crispest action on a Python I ever felt. I tried using one that was issued to me for competition once and it about drove me crazy because I never could get it as smooth as Lt. Jungkinds.
I don't mention another criteria that you may be considering for your purchase - winning a gunfight. I carried S&W revolvers (personally owned and issued) on duty from day one until retirement, a span of almost 30 years. I never had one of my service guns fail me on duty or on the range - and they got used - a lot. But, lots of Coltsters could say that, too.
Whatever you get, shoot it a lot and you'll be more proficient than 90 percent of handgun owners. I hope this helps you in making a decision. BTW I don't own any S&W stock, but I do own a new 460 Mag from the Performance Center - now there's a gun!!