The quality of the management of any resort is what will matter most over the long haul. If a management company does a poor job, it's up to the owners to vote them out and bring in a new management company.
There are enough beautiful old vacation resorts in the world to satisfy my concerns over a deeded resort falling into disrepair. Also, the underlying real estate (land) in a very desirable area ought to retain enough value that the property could be sold or rebuilt as a worthwhile approach, if the buildings were to deterioriate significantly. Either way, some value is retained for the owners. Therefore, I prefer a deeded (owned, not leased) property.
The ownership vs. RTU issue matters to many buyers also. This can become an issue when reselling. I would be more concerned about a RTU, if I didn't expect to reach my "break even point" by the time the resort contract is within 20 years of ending. Resale value probably should not be part of the decision-making process on ANY timeshare since it's always uncertain. With Disney, there seems to be enough of a current market for reselling that this is not much of an issue at this time anyway.
Lastly, I've always wondered whether Disney maint dues will become extra costly in the last 5 years, to ensure that the resorts are returned to Disney's regular inventory in pristine condition at 50 years old. Just speculating.