Currently, the lowest Ruger GP100 4" ss 7 round sells on gun.deals for exactly the same as the lowest 686 Plus 4".
Between the Taurus 66 and GP100, if you can handle you own a gun that is 300 less than the other you were considering, the 66 would be my choice. If you can handle a revolver costing you 650 dollars, the 686 will bring you extreme pride of ownership. They are nice. The triggers are nice. The triggers are no different than previous generation 686s. They can't be, because the MIM parts and lock have zero play on the actions. You just heard that above. It's not a real thing.
Having bought 66s 10 years ago to last year, I assure you, there is no build difference in the 66. None. There is no generation differences, there is no metal differences, there are no part differences. People repeat that Taurus quality changes across years. In the 66, it has not. You might find difference in the 1980s 689, the predecessor of the 66, which S&W was making with Taurus in a cooperative plant in Brazil. But the 689 is still being produced, just not sold in America.
There is truth to Ruger losing some quality starting 2014 when they hired a bunch of people and recently let them (mostly all) go. The Ruger GP100 1771 still is being sold with issues of not all factory ammo fitting in the 7 round cylinder. This makes the 7 shot 1771 a 3 shot if you don't have the right ammo. This error is between individual 1771s and demonstrates the quality variation within the Ruger 1771s. An example in autos, the LCP Prescott AZ is perfectly fine. The NC LCP is having issues.
Also, Taurus just put out the 692 in 2.5", 3", and 6". It is built on the Tracker frame (smaller than the medium 66). It has a swapping 9mm cylinder with moon clips. A thought if you can tolerate the look of the tracker models. I can't.