They're interchangeable, but where I am it is very rare to hear a guy say 'fabulous'. Most choose 'fantastic'. Here, if you're male and say 'fabulous' it can sound effeminate. Whereas for females, I've heard both fabulous and fantastic used. Just something to be aware of.

Oh, I love your explanation.

Yes interchangeable. Fabulous can also mean fashionable. Fantastic means better than great.