The rounded version may provide a smaller (more concentrated) area of contact, but a stronger/better one.
It also seems that it would be easier to insert VS the rectangular version.
If the construction/materials remain the same I doubt that it would make an audible difference in an SET design as I could never hear a difference between unmatched 300B’s (same tubes/different test readings) in an Audion amp and the same with 2A3’s in my Bottlehead amps.
Just guesses.
DeKay