I have 2 theories on how to choose the right mower. First is just purchase walk behinds. There efficient and less expensive. I've noticed the large 15+ truck operation guys only run walk behinds. They are like stated above less expensive. Less to maintain in the fact that they have less moving parts. There also much more versatile then a zero turn.
My other theory is buy what fits your yards the best. You don't need a Z if you cut a lot of hills or un even terrain. But if you have large open areas a Z mower is very efficient. Now to shoot a whole in theory 2. Your accounts are always changing and hopefully growing. Unless you have a guaranty one + year contract you buy a mower for that contract that cost 10k and you loose the contract. You still paid or paying for a mower you no longer need or possible cant use. I know a company locally picked up a park. He purchased 30k worth of 72 " mower. Lost the park the following year. He couldn't use thing mowers on any other property's he had. When he sold them he lost most of his profit.