Aloha,
Denise is correct that there are serious issues with some of the KBV buildings. I would advise everyone to decline to accept ownership of a unit there even if offered substantial money to take it.
An exchange in this year would be a different matter. (The future of KBV as an ongoing operation beyond 2024 is not certain) We've stayed at the Marriot / Royal Sonesta a few times over the last 3 decades - as recently as February this year.
If I were 29 and being offered a choice, I would pick the Marriot KBC unless the following details were very important.
As already mentioned, KBV has a full kitchen.
The access time from a KBV unit to the car is no more than 1 minute. The parking set up at the Marriot has many variables. Unless a spot is found in the east public access lot, time between unit and car can be many minutes. If making more than one car trip a day, this could be important.
KBV units have laundry machines in each unit. I'm not sure about Marriott KBC.
KBV is located on a very sparsely used non-swimmable beach over a mile in length - much greater privacy.
KBV has access to the adjacent hotel pool complex. Not as fancy as Marriott KBC but still pretty good.
Unless the exchange into KBV is for 2 bedrooms, expect a parking lot view. The 2-bedroom units that exchangers generally wind up in have a lagoon view.
Jack