I am new to the site and am getting ready to purchase a new ExMark. I am looking at 20-23 HP, 52" deck models. I am a residential homeowner with 3 acres of flat lawn to cut, I currently have a 38" lawn tractor that takes about 4-5 hours to mow, so I have decided on a new ExMark, but I can't decide between the "Z" and the "HP"! Can you help explain: What are the major differences between a full-size "Z" and the "HP"? Why would someone choose the "HP" over the full-size "Z"?