Tunney. Gene would box cautiously,avoiding a lot of Fitz's much vaunted body shots. Tunney by decision.
Jack McAuliffe was asked about this, and he stated that Tunney would win because he took and gave a punch better than Jim Corbett. Just one opinion.
A valid one too. Personally I think Tunney was Corbett, but vastly improved. More durable ,and better power.
Tunney looks basically modern except for the low hands. Corbett, on the other hand, is a marvel of the older, pre-gloved style. Each guy was as near technical perfection as he could be, but they used completely different textbooks.
Johnson rated Corbett very highly,certainly higher than his other near contemporaries. He rated Dempsey above Tunney however.
Curiously, McAuliffe implied that they were more or less equal as technicians, but said that Tuney was more durable and a more punishing hitter.
No HW ever took Fitz 20 rounds pre-Jeffries, did he? I accept that Tunney was extraordinary and I agree that he comes from a different era in terms of aspects of technique, but I think either Seamus is right or orriray is right. I'm not buying this boxing-carefully-to-a-decision cool-aid. Fitz was a monster. As the OP likely knew, this comes down to how you feel about boxing's evolution.