Your speed on either machine is likely to be determined by how rough or smooth your lawn is. I've mowed my 2+ acres with both the JD 62" and the Exmark 48" WB. I'm never in any real hurry and have never mowed at full speed which is probably 6-8 MPH for both machines. If a tractor doesn't seem to be a good fit for you then get the zero turn. It will at least save you some time in that you can turn it around in a heart beat.
....how we view life can sometimes be effected simply by which side of the grass our feet are on......