I never used the bus but the train or car, indeed the car route is scenic so I can only recommend it but the train route also is...I like train trips though.
Your friends are right, Ronda is a MUST visit place, you will not regret.
About other recommendations, I would not miss a visit to Mijas Pueblo...and talking about Ronda area I know you are choosing between bus or train but for others who will read this, if anyone is hiring a car
I recommend to explore the surrounding area and visiting other villages like Ardales or El Chorro.
For more ideas I just wrote a blog post yesterday about ten places not to miss when visiting Malaga (Ronda is there), just need to translate it to English, will add the link here when finished.