I am not a property law attorney, and for a real answer you probably should consult one. But to my lay understanding, UDI is having multiple people on a deed as owners of the property, except with the amount of interest strictly laid out. UDI means you have the specified interest in the property as an owner, and the administrative stuff makes it possible to work out how to express that in terms of weeks in units. But in Wyndham, it's no specific unit or date, just interest in the resort. I guess this is sort of like the old silver reserve dollars.
RTU from my understanding is really like joining a golf club or something - it's at best a sort of lease - you have no actual ownership at all. Just a contract for use at the schedule etc laid out. This is more like a fiat currency, there's nothing behind it. The upshot is you avoid all the deeded property transfer hoopla.