How much better? You want a quantifiable answer?
It will be 23 betters.
I'm running 255/55 Wrangler on 18inch Hurricane (standard factory fit) with no problems what so ever.
Road handling will get worse. Off road handling will prove so will comfort. Depends and what better is for you.
Getting rid of the 20's is a no brainer tbh. Wins in every direction.
You could also go up to 255/60/18.
Slightly bigger side wall and will fill the arch slightly better andcalso make your speedo read spot on.
apart from road handling, I.E cornering, braking, accelerating. They will all suffer