What about a third combination: used GT (like a JD455) and the 40hp tractor. GT's are more versatile than zero turns, and if you get one with 4 wheel steer it'll cut just about as fast as a zero turn. The GT can be equipped with tillers, plows, garden cart, etc., and fit into places the bigger tractors cannot.
I have a JD 425 AWS and a Kioti CK25 TLB. I use the CK for heavy lifting, serious ground engagement. I use the 425 GT for mowing, snow plowing, and light garden towing. I like the GT better for snow plowing due to tight spaces around cars, buildings, etc., and I can push sideways in an 11 foot wide driveway; can't do that with the CK. The CK stays off the lawn almost entirely to prevent rutting and compaction.
Of course, for similar money a used SCUT is equally as versatile. My view of SCUTS: Not the ideal mower or ideal earth mover, but is a do-all machine, if you really need for just one machine to do it all.
Basically, it depends on your topography and exact uses. Tight spaces and a lot of low hanging branches points to a GT/zero turn. The more open your space the more usable a bigger tractor becomes. The big tractors require more space and are hard on your lawn especially for mowing, and the little tractors struggle to move much dirt. On the other hand, if you don't need a great big tiller, and don't have many other earth engagement projects, then perhaps a high end GT is all you need. A good used JD455 (and similar) can be had for under $5K, and there are many very effective, high quality attachments for it.
The zero turn or a commercial walk behind are excellent choices for mowing, as long as other "garden tasks" aren't needed and your land conditions allow for such machines.
But to your original question, I'd opt for the latter. It's just much more capable.
