Like it says in the title… the main difference I’m aware of is that include(CTest) has extra features for integrating with CDash and defines the BUILD_TESTING option.
But should one be preferred over the other for new code?
include(CTest) adds a bunch of targets that can be used to run parts of a dashboard submission. If you don’t need those, enable_testing() may be preferable to avoid the extra clutter. I used to use include(CTest) a fair bit, but these days I almost always just do enable_testing() since I use a dedicated dashboard script if I want dashboard submissions.