I'd go so far to say that not only was Mystic Gohan stronger than Gotenks, but he was stronger than SSJ3 Gotenks by a small amount, although SSJ3 Gotenks never really used his full power against Super Buu because he was always messing around and was unfocused so it's a close one to call. Something notable was the way Super Buu reacted to the punches and kicks of both fighters. With Gotenks he recoiled a lot, but with Mystic Gohan he literally changed the shape of his body, like SSJ3 Gotenks' Super Ghost Kamikazi Attack did.