I have some lawns where the 44" wb finishes as fast or faster than the 48" ztr. Usually it's small lawns where I'm turning a lot. You have to slow so much with the Z that you lose time there, especially on thin turf where damage is possible. But on other lawns the ZTR blows it away.
You also have to factor in the ability to mow backwards when needed, the quickness of reversing in and out of spots, etc. most ZTR's also have quick height adjustments that allow you to change height "on the fly". It comes in handy here with varying mowing heights, and also allows me to fine tune cuts over crests and other areas where you need to raise the deck slightly to avoid scalping. Most if not all wb's can't do that.
But most of all, you have to remember that standing on a sulky or walking is a lot harder on the body than sitting. And in my case at least, I have more time than energy anyway.
I also find that I don't mind making a high speed 2nd pass over a lawn to disperse clumps on the ZTR, but it would be more of a drag doing that wiht the wb.
If mowing part time, a wb would be fine. Full time, you probably need both types or will have to turn away some types of lawns.
Also, it's hard to time mowing exactly since conditions can vary week to week. But say you're doing a typical 1/4 acre subdivision home. You might only be mowing for 15-20 min. Say one type does the job 10% faster. You saved 2 minutes. Time is money, but other factors begin to matter more, such as cut quality, turf damage, fatigue, etc. Of course it matters more on bigger props and when doing high volume stuff.