In these "old vs new" discussions, "better quality" usually refers to the fit and finish of the older guns. New guns might also have The Lock, but they shoot as well or better as they ever have.
As far as the lock, here are some thoughts from some of the best competitive wheelgunners out there. You can draw your own conclusions. Seems particularly relevant, since you mentioned IDPA might be in the cards at some point:
http://www.brianenos.com/forums/index.php?showtopic=110071
And regarding IDPA, a few other random things to consider:
1. The pinned front sight of newer 686s are relatively easy to swap out. Many find a fiber optic front sight much easier and quicker to see than the factory red ramp.
2. IDPA mandates that all original safety equipment must be on the gun. This includes the internal lock mechanism, so if the lock is an issue for you, buy pre-lock, rather than removing it from a new gun.
3. Older guns utilize hammer-mounted firing pins, whereas newer ones use frame-mounted pins. Taking the pin off the hammer lightens the hammer a wee bit, and IMO, there are some benefits to a lighter hammer. A newer gun, converted to DAO can utilize a very light hammer, and it's here where its benefits really become apparent.
4. The 686 is now also available as a 7-shooter. The extra capacity might be attractive, but you can only load to 6 in IDPA. You can
use a 7-shot 686 in IDPA, you can only load (and reload) to 6, so you'd actually be handicapped with this gun.
5. Compensators, including ported barrels aren't IDPA-legal. Something to keep in mind when shopping for a used gun.
6. The 586 is the blued version of the 686, so you might consider it as well if you see a nice example.
7. S&W currently offers the 686 SSR. As the name suggests, it's another viable choice for IDPA/range use. You can likely buy a standard 686 and have it tuned & customized by a good 'smith for less than the SSR, but if you didn't want the hassle of getting it to a gunsmith, the 686SSR might be a good choice.
http://www.smith-wesson.com/webapp/...57775_757751_757751_ProductDisplayErrorView_Y
Finally, if you're looking at used revolvers, check out the Revolver Checkout thread:
http://www.thehighroad.org/showthread.php?t=1430