I own several of both models, with varying barrel lengths. I'll base all my opinions on the 4" barrel length of both models.
"best", being subjective, I'll tell you which model works best for me, and why. If you can arrange it, you should really shoot both, as there are slight differences between the two models, which while they may not matter to me, may be of consequence to you.
I use my 4" 66 for range and HD, and carry a 2.5", and 3" for CCW and work, on the occasions I carry a revolver. The 66, and K-frame magnums in general, are lighter, slimmer, and just point more naturally, for me. All of my model 66's, have been VERY accurate. I would call my 66-4 3", and 66-2 4" tack drivers. 2.5" at 20 yards being fairly routine, when I do my part. The cylinder is narrower on the 66 which makes it easier to conceal as well.
I have a 686-3 4", a 686 CS-1 3", and just put a 686-4+ 4" on layaway. These are beefy revolvers, IMHO. Being L-frames, they are heavier than the K-frames, and wider in the cylinder. While they are well balanced, they are just not as "lively" as the K-frames, for me. I've not shot the 686-4+ yet, but my other two 686's are accurate revolvers. The best group I've shot, was with the CS-1, being 1.75" at 15 yards, using WWB 38's.
Hope this helps. Good luck with your decision. Regards 18DAI.