Keep stock 20" wheels on a Platinum or swap back to 18"?

probably not a whole lot different that the bfg ko2 255/85/20's i've ran for the past 60,000 miles. all terrain tires are louder on the highway than your run-of-the-mill all season tire but nowhere near a mud tire. you will not be disappointed

 

Ok thank you, I think you probably meant the 285 55 20, sounds like compared to my current Toyo OC AT2 P275 60 20 LR B, they might be just a bit firmer.

 

I vote keep the 20 inch wheels OP. Just get a 275/60/R20 or 285/55/R20.

 

Why not 17"? Honestly comes down to your priorities. I can't think of a single reason the 20" is the way to go unless you prefer the aesthetics, need to clear some ridiculously large brakes, or you love some specific option offered only in 20". I have to assume the only people that care about "larger rims" are other people who spent money on their large rims. In my opinion smaller wheels = better ride, less rolling weight which equates to more performance accelerating and decelerating, tires cost less, you can air down more if you off-road, you have more clearance from rocks or curbs as you have more sidewall.
Whatever floats your boat as at the end of the day it's your truck to drive and look at. Good luck!