What is the difference between “option” and “set CACHE BOOL” for a CMake variable?

In your example, there is no difference. But there can be differences:

• option has a default value of OFF.
• You can make an option dependent on other options with CMakeDependentOption
• set accepts types other than BOOL, especially ;-lists.

Additionally, when you use an option you are indicating to your user that it can be set safely. Setting internal variables from the outside might subtly break the script with invalid values.