i upgraded from a 18.5 ots to a 22.5 otg last year and have been happier with the bigger real estate. 1 chimney is plenty for me to grill a few steaks but what i really like bigger grill for is indirect cooking. you can get a rotis for the 22.5.
don't get rid of the 18.5 unless you have too. it's still better when you need a really hot fire.