When I visited Spain in March of last year, I took the AVE train to Sevilla to Cordoba as a day trip. It was a fast and comfortable ride in 2nd class.
I also took the bus to from Granada to Madrid.
THAT WAS THE WORST BUS TRIP IN MY LIFE! I took the early Sunday morning bus to Madrid. The bus was packed with people, there was hardly leg room, my seat was uncomfortable to sit in and the bus play Terminator III as the Sunday morning movie. The volume of the movie was louded and I could not fall asleep until it finished.
I been to Spain twice last year and that was ride was my worst experience from my last five years of traveling. I would not recommend the bus from Granada to Madrid.
If I have to do it again, I will take the train from Granada to Madrid. At least you can get out of your seat and even stretch your legs on a long train ride. Better yet, I can take a take a bus to Cordoba or Sevilla and the pickup the AVE to Madrid. At least on the AVE you can travel in comfort.
Bill