I first bought a ZD21 in 2004 and used it to mow about 5 acres weekly around the farm. Good machine. I decided to upgrade this year and am very happy with the ZD331. It has some upgrades that were annoying with the 21. Foot pedals for the lift and e-brake. Easier access to the deck for greasing and service.
I hate the dual tank system. (2) 6 gallons. One tank has use of the fuel gauge the other has a light that starts blinking when it gets down to half a tank and then you change a valve to the tank with the gauge. They need to connect the two tanks so the gauge reads both like semi trucks do.