I actually agree that Von has been better than Sonny recently, probably since as far back as the late 70's. There's just not enough recorded material around to know about earlier than that.
Though he has demurred on this point in interviews, Von's rep among his fellows, including Sonny Stitt and Andrew Hill, was that he was the best tenor to ever come out of Chicago-- a strong statement that Von's recordings, particularly Young and Foolish, Serenade and Blues, and Good Forever certainly lend credence to.