"Practical" difference, I'm not sure what your end game is here. I have owned both and each has their draw.
If you want panache and scarcity, the Colt King Cobra wins here. 686's are everywhere and growing. The Colt will likely cost you more but will also appreciate in "value" more becauae Colt snake guns simply do this and cannot be stopped.
If you want to shoot them, the 686 has a better double action trigger stroke and is far easier to tune for an even better trigger. And the actual contour of the 686 trigger is far more friendly to my trigger finger than the grooved trigger of the Colt.
Both are robust and durable and unlikely to fail... but if something does go awry, I don't believe Colt is going to be much help while S&W still services any/every 686.
Lots more grip/stock options for the 686 than the KC, if that matters.
I sold my King Cobra and while I thought it was a gorgeous revolver, it just wasn't nearly as good a handgun as my 686. But at the same time... nobody is ever gonna give me the same kind of money for my 686.