If you rephrase the question to “Which is LESS harmful to the OT?”, I would say the Sequels. The Sequels are unnecessary supplemental stories that can be easily ignored. But the Prequels are vital backstory to the OT, and are now inextricably intertwined with the OT in the public consciousness.
Let me put it this way: Jar Jar Binks now appears in Return of the Jedi.