Corbett just off (May 1900) a very honorable loss to champion Jeffries, many saying the best he'd looked since he'd fought Sullivan in 1892 -- some pointedly adding he'd showed better form than when he'd dropped the title to Fitzsimmons three years earlier. Fitzsimmons, needing to reestablish after being fairly handled and stopped by challenger Jeffries a year earlier, KO's favored Ruhlin, then a couple weeks later ices Tom Sharkey (both in August 1900). (Tons of boxing activity as fighters match up before the New York Horton law's repeal, limiting boxing to members clubs not charging admission -- making big east coast money no longer available). Corbett-Fitzsimmons II -- something Fitzsimmons told Corbett would never happen -- seemed an obvious match-up. Could Fitzsimmons 'retirement' -- fighting no contests, but keeping in relatively good shape while sparring/exhibiting -- have been -- in part -- a way of quashing talk of a rematch with Corbett (while also sitting on his outstanding challenger status, and also wait-and-see'ing how the (more lucrative) New York boxing situation played out)?
Between this pair I would give the benefit of the doubt to Fitz here. Part of why that was the best Corbett had looked since beating Sullivan is he'd barely fought since then. Fitzsimmons gets the credibility here. Hes the one who destroyed Ruhlin and Sharkey and fought at a high level for a much longer period of time, much more frequently at all times.
Old timers seemed to regard Fitzsimmons a bit more highly; with some exceptions. Actually, both Bob and Jim had a fair span between their notable accomplishments (calling the admired challenge of Jeffries, and the defeat of McCoy, as being among Corbett's accomplishments). But, yes, Fitzsimmons has more 'filling'.