I often thought the other day I'd like to have a 20... but then I'd be jockeying my 15' in and out of a narrow spot and realized 20 would probably not save me any time in the long run.  If I was going to go bigger, it would probably be a Schulte.  Neighbor has a 20' Schulte on a 225 horse deere and he isn't kind to the mower, no problems in 5 or 6 years.