It's probably complicated. I'm curious, too. Obviously, the Abarth models have 5-speed manuals overseas, but those also have higher torque, which I believe was the primary reason for the different unit. Keep in mind in Europe, I believe that a 5-speed manual is usually standard, and that the 6-speed is an option. Out options are still rather murky (at least that's my hope!) so who knows, maybe an optional 6-speed? Not likely, especially since most Americans can't even drive stick.
That said, I'm used to 5 speeds, and it's more about the gearing and spacing, in my opinion. Come on Fiat, call me already!
398 -> 24