17" is cheaper, you got that right. And you'll also have more choices in a 17. Pretty much all the major and most of the minor manufacturers make 17" sizes, so you got that right too. I also prefer a C or D rated tire for ride quality and sidewall compliance. I'm not aware of any 18" truck tires that are NOT E-rated. My ride quality improved soooo much when I ditched my E-rated Nittos and got D rated Mickey Thompsons.
Also more sidewall translates to more aired down compliance and less risk of pinch flat (though at 37" tires, not much difference at this point). More sidewall = more big truck tire looks
As Heckraiser said, 17's are cheaper, you have more options, and in addition to D rated options, you can get the C rated 37x12.5R17 BFG that comes on the Bronco Raptor. Yes, yes, its smaller than some other 37's but its taller than any non-bias ply, non racing 35, AND it is the lightest 37 inch tire on the market (65-66 pounds vs 75 or more for most other 37's). That makes a difference when you accelerate, turn on the road, and brake... and it puts less strain on all of the parts you pound on when your offroad.
Now... I have still looked at 18 inch wheels and even (gasp) 20 inch wheels, for some builds I've helped with. Why? @TeCKis300 covered the case for 18's above pretty well. And as an aside you can get some D rated 37's for 18 inch wheels, but, only the 13.5 in wide 37's, which, while looking cool, are probably a bigger PiTa to set up on the LC200 to tuck inside the fenders. On Jeeps and Bronco's that's less of an issue.
As for a 20 inch wheel... I haven't run then on a personal rig, but I did help with one build with a BBK (that made sense for the weight of the rig and speeds the owner liked to do on road
) and with a 37 inch tire, you still have a reasonable amount of sidewall. Horses for courses, for builds, and for personal preference.
