Fitz. Foster has a shaky chin vs power hitters. Fitz crushed far more durable heavyweights than Foster.
Foster, boxing progressed light years, you don't do this against Foster and expect to not get ko'd early by a left hook http://coxscorner.tripod.com/Images/fitzevasion.jpg
Foster was a big puncher at 175, but his power was not there at heavyweight. Foster had a shaky chin, and his comment that the heavyweight jab felt like the light heavyweight knockout punch tells you everything, Foster was Ko'd by Doug Jones, and Ernie Terrell, neither man was a big punching heavyweight. Fraizer was, and Foster lasted 2 rounds. Fitzsimmons had his chin tested by the likes of Choynski, Sharkey, and Maher. None stopped Fitz. Only Jeffries did, and it took him 11 and 8 rounds. So unless you think BobFoster hits much harder than the above names, he's not taking Fitz out early. By contrast Fitz flattened more durable men than Foster.
None stopped him, but all dropped him.:think I'd say Foster hit harder than Corbett who also dropped Fitz. Fitz was very vulnerable to the left jab, Foster had a very good one. I pick Fitz but he might taste canvas first. Amended: Foster stopped quite a few of the heavies he fought ,his problem was more that his stringbean build meant he could not absorb a heavyweights punches too well. Foster was facing heavies who for the most part were significantly heavier than those Fitz encountered.
So can I Bottom line is Foster didn't have the chin to beat the best punchers. Smaller heavies who did not punch hard sparked him. Not having a good chin vs Fitz is quick way to lose. I do not think Foster hit significantly harder than Maher, Choysnki, or Sharkey. In fact he could had less power. Each man tested Fitz's durability. Keep in mind back then the gloves were light and there was no gum shield.