I recall a Ring article about this in the early seventies that asked the question, Could Sonny beat a bigger more mobile version of himself?
i wouldn't say more mobile. Bigger and more powerful though. Liston was definitely the better boxer Imo.
http://i908.photobucket.com/albums/...EATNESS/SonnyListonGeorgeForeman-Sparring.jpg Liston sparred Foreman pretty regularly and always got the better of him, but Foreman would have been VERY green during this time period.
I'm actually leaning towards Liston. They're close in power, and Foreman has more heart, but Liston had the edge over a pre-retirement Foreman in stamina and chin. A Foreman victory wouldn't surprise me a lot though. I actually see this playing out similarly to the Foreman-Lyle fight. It mainly depends on who can land the knockout punch first.