I've never shot the RF version, but from everything I've seen/read/heard about it, better IQ is not a major reason to go with the RF. Both the latest EF and RF 70-200s are superb optically, and differences are splitting hairs. Can guarantee in a blind image test nobody could accurately pick one over the other.
Pay the premium for the RF if you want the smaller size and faster AF.
Buy the EF if you want to save some $$ and don't mind the extra bulk.