Over these many years, I have usually preferred to stay on property when visiting Disney World. The benefits were easy to enumerate; including transportation, quality, extra access to parks through early opening and e-ticket events, merchandise delivery, and the attention Disney gives to the little details of your stay. My few offsite experiences were certainly positive, but did not approach the wonderfulness of staying in a Disney Resort.
That has been changing.
I will say, with little doubt, that Disney's premium resorts are a true wonder. How do they compare to offsite facilities? Well, I think that most folks who stay in these resorts aren't too concerned about saving every last dollar; the expendable income is likely to be a little higher. However, I have noticed that offsite resorts are increasing the amenities and benefits they include while Disney seems to be cutting back.
Merchandise is no longer delivered to your room, but it is still sent to the resort. You usually have to go to a central location and pick it up. This works better when the castmember actually calls you as they are supposed to do.
Transportation has been going downhill overall. Premium resorts should provide premium service, not so with the buses though. I have consistently spent more and more time waiting for the correct bus to arrive and it is not unusual for it to be SRO or completely full.
Dining is great in the sit-down full-service restaurants. Not so great in the food courts. Overpriced for mediocre quality. Disney is not like this all over. At the Boardwalk you can get a great hotdog for $3.00 and I have even had one for $2.50 in the Magic Kingdom. But, $9.99 for what amounts to a few chicken fingers and frozen fries?
Accommodations are good, but they are good at most nicer hotels in the area. TV channels are a little limited and the amenities like ice, vending, and even elevators are typical for a nice resort. Not necessarily outstanding here. On my last trip, we experienced an ant infestation (on the 4th floor), a leaking kitchen sink (2 times), a non-working HVAC, and a refrigerator that would only make things cool -- not cold. Service appeared right away after each report, but it took a conversation with the housekeeping manager to make the fixes permanent.
Staff is an interesting area of comparison. Disney castmembers used to be the absolute best, all going out of the way to make you feel really welcome and comfortable. Many still do, but the hires of the last few years are a different breed. Perhaps it is a cultural shift due to adding a higher percentage of college program staff and immigrants, but the bright-smile and welcome just aren't as pervasive as it used to be. On the other hand, the quality of staff at offsite resorts is generally a step below that, making the experience just a notch lower on the happiness scale.