Cooper says that tank v tank - the M-26 was only on par with the Panther, not the Tiger.
It was the close in armor protection with the Panther - the 90mm M1 gun had a lower muzzele velocity than the 75mm on the Panther - which had a slight edge in penetration capability. In one incident, the projectile from the 90mm struck the Panther directly on the glacis plate at less than 300 yards range and ricocheted.

It's interesting that although the Sherman was supposedly more "mobile" - although it was a lighter tank in tons - it's ground bearing pressure was higher than the Panther/Tiger - and less mobile over snow and mud. (those damn narrow tracks) Cooper sums up that the Sherman was deficient in almost every area - from armor protection to gun - and he blames its failure (and the lack of foresight by certain people) for the deaths of many, many Americans.

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"I die for my fatherland. I have a clear conscience." - Admiral Wilhelm Canaris