I have a overland 2012 with 20" and no trail rating. I've taken it on trails.. Will you? Just wondering, not trolling. 20's work off road with good rubber, but more likely to get damaged, and less cushion.
I do have 18" polished oem rims I put blizzaks on, so I know how they feel compared to 20". My 20's have terra grapplers. The 18" feel softer, smoother, a bit less responsive. Overall, a better ride for a SUV, but the blizzak "shaggy" tread helps too. Of course they are less precise than the 20's.
FWIW, I'm looking at a yukon next and may go with a tahoe z71 with 18's because I like more rubber, softer ride for an SUV. When do you ever get even close to the limit on road in a SUV? whereas, the z71 has WAY more off road ability than say a yukon denali (8" clearance!)...
Go either way. I put plates on myself (allowing me to put on undercoat on the upside of the plates to prevent rust) and front hooks. Neither was that hard, but it was more money than a set of 20" non-oem rims, I assume. Hard to believe OEM 20's would run 800$ used. I got like new 2012 polished oem 18's for 300 with tpms!. If you need winter tires anyway, consider rims and tires as a set too.
Honestly, if I were going to keep my JGC I'd trade my 20's even up for your 18's because I'd rather have 18's anyway. I can't be the only one thinking that. 18's are cheaper tires, and many more options to choose from, better off road, and (a touch) softer ride.
Can't believe GY (forteras were ok at best, not good in snow) are better than michelin either, my 2c...
Good luck.
I think the 2016 JGC is a good vehicle to buy, fwiw.