I thought about including this data in the book, but concluded it detracted from my main point--that Greenleaf made pool popular and the sport's popularity quickly faded without him. I didn't find a single source who said that Mosconi was the better player. But everyone loved Greenleaf so those recollections are biased. Billiards Digest (Dyer / Ursitti) did a good comparison of the data here:
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But I will note that Greenleaf played on 10 foot tables against must tougher competition than Mosconi later faced in the post-WW2 era. Most of Mosconi's tournament victories were on 9 footers played in invitationals with only a handful of players. By WW2, when Greenleaf was in his mid-40s and Mosconi was in his early 30s, Willie was certainly the better player as shown in their 1945 tournament showdown. I think 1937-1940 is a better comparison as Greenleaf was in better health (he was in his late 30s) and Mosconi was a full adult (mid 20s). Greenleaf dominated Mosconi during those years and didn't lose a single reported match to him.
Of course, if you asked who to bet on, I'd say Mosconi. Willie always showed up on time and in condition to play.