well, my truck came with 20". Upside is the shorter stiffer sidewall seems to give better stability than the 16's on my older truck. Downside it number of available tires. not much to choose from. And NO true winter studdless snowflake LRE tires available. To do that I had to go down to 17" on another set of wheels.
Upside of 17 or 18 is more tire choices.
Price wise, 17 or 20's of similar tires seems to be pretty close. but the price RANGE of tires in the 17 and 18 has lots more low end tires.
for my 3/4 ton 17, 18, 20 are all factory option tire sizes with very close to the same load rating and same overall diameter. aspect ratio just changes with tire size.