It completely depends on what kind of terrain you have. Wide open flat turf, a Z will do well. Small, hilly terrain and a Z is useless. The standers are very comparable to a Z. Currently we are running 5 walk behinds , a Vantage , and a Laser.
The Vantage is for sale and the Z gets used 1day a week.
A lot of guys run a Z simply because it is easier. In my opinion, putting a Z on a 5000 sq ft lawn is a horrible idea. But if you need to get the work done fast and aren't really concerned about how it looks then a Z is the way to go.
Look at the big companies. What are they running? For the most part , walk behinds, there is a reason for that. They are more versatile.