Holy cow, that's a lot of seat time. Choose ground speed over width of cut (assuming it's decently smooth). Ground speed wins every time on something of that size.I am looking hard at the Toro / Bad Boy mowers currently. I mow about 6 acres and have been using an MTD 50" regular riding mower for most of the time last couple of years...…I borrow a A zero turn from time to time and it cuts the job in half time wise and works so much more efficiently. Initially, I couldn't justify the extra cost but now it just makes to much sense not to upgrade. Life is so dang busy, I have got to get something that gets the job done faster....
Is that actual lawn or just field or something like that? That size you could almost justify going to a tractor with a decent bush hog on the back. It won't be any faster than a ZT, but it will ride a lot smoother and you can use it for other stuff. With six acres, you have plenty of other stuff.
Then again, a good ZT could mow that in 3-4 hours, give or take. Minimum 10mph ground speed for something like that. Stay away from big block (1000cc) engine or you'll be burning 2-2.5 gallons of fuel per hour. That's why they have the ability to carry ten gallons of fuel. 27-30hp should do the ticket.
Oh, and don't mow it with the deck on the ground. 3.75" minimum. Much easier on equipment, fuel usage and the grass itself.