I think Moby's old set of 40x15.5 mud grapplers on 20" slabs looked bad ass! It was such a wide stance it was amazing. If your not going to run a set up like that I think 20's are to big. Maybe 18's with 37's. It's really up to you tho and what like your going for along with how your gonna use your jeep
I have 20's with 35x12.5 and have been thinking about trading for 17's. since 80% of my driving is mall crawling I haven't pulled the trigger yet. Mainly because the local trails here are easily do-able with my current setup. Half the time i dont even put it in 4wd. You could seriously do trails here in a subaru. Now if I ever venture further west to some real trails and rocks I would definitely run 17's for airing down and rim protection. And yes bigger tires look better than bigger rims on a jeep. If you check prices 17's are defiantly cheaper.