I agree with most of what David said, except on not using it as often as you think or not being faster than your 21". I have one terraced place on the mountain here that I can't even fit my 36" on, if it wasn't for that it would never come of the trailer. It would just be a backup. The additional sizes makes you more flexible, which allows you to offer more services or bid more jobs if the need ever arises. I don't pull anything off my trailer but my 60" or 66" mower, trimmer, and blower, unless there is a gate or something blocking my way to get to the property that needs mowed. If it's 10' x 10" my 60" is three swipes and done. Takes as long to back off the trailer and reload as it does to cut it. You'll love a 36" over the 21". Times that love by 10 going from a 36" to a ZTR. I'm by know means saying buy a ZTR or anything out of your budget or need, but with the right tools you can do amazing things.