I expect there is some efficiency to be gained in switching from a PTO to a dedicated generator. In my case, the fuel consumption of an 18 HP dedicated unit is considerably less than the 60 HP tractor. So far, with my test runs, I'm saving around .8 gal/hr. while generating the same KWH. This fuel savings may not amount to much with the power outages we get here but there are other factors.
I suspect a larger tractor engine does not work as hard as a smaller one, and conceivably would last longer, but the running hours are a consideration. Sure, spinning a PTO generator is easy duty for most tractors but an hour is still an hour when it comes to maintenance and resale value.
Changing 8 quarts of tractor engine oil and 12 gallons of hydraulic oil, plus filters every so many hours will be far more expensive than one gallon of engine oil and a single filter on a generator.
It's tough to put a dollar figure on the loss of use of the tractor for other purposes and the hassle of hooking up the PTO, but it's certainly worth something.
Of course, all this depends entirely on the number of outages you get and how long they last.