If you are looking at older tractors, you want to look for a "live" PTO.
On older tractors (1960 and older for debate), you would push the clutch in to change gears to stop the tractor, and you would also remove power from the PTO. For example, you are pulling a baler. You get into some heavy hay and need to stop the tractor to let the baler catch up. If you do not have a "Live" PTO, the tractor will stop along with the PTO.
I can not think of any modern tractor without a live PTO. During the time before they figured out the LIVE part, many tractors added a hand clutch in addition to the foot clutch. In this set up (which my tractor has) I would use the hand truck to stop power going to the wheels, while the PTO will keep spinning.
As others have said, there is NOTHING (debatable) more dangerous on any farm than a PTO.
As others have said, there is NOTHING (debatable) more useful on any farm than a PTO.