Not instead of. It says more. A person who raves about someone speaks constantly about them with uncritical praise. If someone says that someone else 'raves' about her husband, you may not want to meet him (except as a matter of interest: 'What is this paragon really like?)
Interestingly, if you "rant and rave" you are saying negative things.
If you rave about your new colleague, you like her a lot.
If you rant and rave about your new colleague, you dislike her a lot.