Neither so far. Randomly numbering things has been totally unhelpful, and left a lot of people confused about inconsistent nomenclature.
I think calling it what it is, is the most helpful. It's the River in the 20th Anniversary Edition. Or the 20th Jubilee River. It's not available separately.
Well, the nomenclature itself is not inconsistent. There is River I and River II, that's it. There are numerous versions of those two, but no River III, or IV, or anything past II.
This new River is the only one that would warrant a new name due to brand new layout and tiles + other factors, but so far it is called just The River in the official rules.
I agree with the names
River 20th Anniversary Edition,
20th Jubilee River.