Comparing the documentation for the new Catenate function and the updated Join function, the pros-and-cons aren't immediately clear. They both operate on both Lists and Associations, and both do approximately the same things, though the syntax is different (Catenate takes a list of arguments, Join takes a sequence). I'd be grateful for some use cases for the two that highlight similarities and differences so that I'd have some guidance about when to choose one over the other. Documentation is found in the Documentation Center under the following paths:
Catenate is the equivalent of Join but for Associations. Of course both were designed to interoperate as much as possible. The idea was to make the functions easy to use.
Catenate is useful because it always returns a list of simple objects. Join is a simpler operation which may return nested expressions containing other Lists or Associations.
If you're not using Associations, jut use Join. If you're using Associations you'll find Catenate handy.