Jack McAuiliffe saw them both. He apparently thought it would be a very competitive match.... "Much talk continually goes the rounds as to the fighting qualities of past and present day champions. I maintain, and will always stand by the statement, that the old-times were far superior as fighters to the ones of the present day. I was always a great admirer of John L Sullivan and I think he was one of the greatest. Since I've met Dempsey, however, I can say that, with his tiger-like fighting methods, he and Sullivan would have made a great fight could they have met."
Jimmy Britt picks Dempsey to beat Sullivan as being quicker to the punch with his left than with John L. with his right. For what it's worth, he picks Dempsey over Johnson, Jeffries, Corbett and Fitz also.
Dempsey if by modern rules. Sully if by LPR or some kind of mixed rules. Sullivan doesn't know how to fight in a modern boxing match, his version of queensberry was LPR rules with gloves. Dempsey doesn't know about the scratch or rounds are down or even flopping. Hell, Dempsey hovers over down men most of his career and struggled to adopt the neutral corner rule ... folks say anyway.