I own a 36" Hustler Trimstar. It fits through gates and is very productive in open areas as well. It is hydro. It also is not a pistol grip. I have operated pistol grip before and with the Hustler there is no hand fatigue.
Think of it this way. If you have a lot that is 50' wide and you run a 36 it will take you right at 20 passes to cut it. A 48 will cut the same area in 16 passes. That same lot cut with a 21 will take 36 passes. To me it was worth more to replace the 21 with the 36 than keeping a 21 and replacing my 42 ZTR with a 48. Only you know what yards your service and what direction you want to take your business. You need to look at all the facts and make a decision that best suits you.