With "Import project from external model" option, you will have the possibility to import an existing project which was created in a different model. Like Eclipse or Maven project, etc. IntelliJ IDEA will interpret the project files (i.e. you eclipse project will be migrated to IDEA).
The "Create project from existing sources" option, will just create the IDEA project over an existing source code.
There is no golden rule. It's just simply which case you need. If you have an Eclipse or Maven project and you want to load it with IDEA then use "Import project from external model".
Otherwise, if you have just a bunch of source code and you want to start fast, then use "Create project from existing sources".