Neither is better IMHO, but one usually fits the needs of each individual better then the other. RCI's fee's are to high for me plus I don't feel they have as many quality resorts as II. They manipulate their TPU's i. Their points system and I don't want to have to work that hard to figure out how to get what I want at a cost I'm willing to pay, this we haven't had a personal account with RCI in probably a decade.
II has also started finding new ways to remove cash from my wallet. I still feel like they have a greater number quality resorts or resort groups, although they have fewer resorts than RCI. Still, we'll be dropping our personal II account when our current membership runs out.
Instead of using exchange companies, we prefer to own in systems that provide us with what we want. Granted Marriott utilizes II for "internal" weeks exchanges but, the II account is part of the MVC club membership fees and it's still working well for us.
Ask the same question in another 10 years and the answers will likely be very different. The playing field changes frequently.