Robert, I have reached a surpising conclusion, a zt, on normal sized properties, is not necessarily the fastest way to mow-that 3 point turn that you really have to take in order not to skuff the turf takes up time and I don't believe that, except for the fastest mowers you can make up the lost time in turning.
For that matter, can you even use the fast speeds of alot of zt on anything other than manicured grass with no or very slight bumps?
Now 6 acres, depending on the sq ft of grass may change things, then again it may present even a stronger argument in favor of a cut/or large gt.
At any rate, if the op goes zt the machine will have to be upper commercial quality to hold up-big engine, big deck, big hydro's-no powder metal here he needs the real deal.
I'm thinking a Kubota 'B' / Cub Cadet Yanmar or Mahindra which is alot of diesel tractor for the cash, an awful lot of tractor for the cash.
Wish I had his 'problem'.