My father always taught me to pump gasoline as slowly as the pumps would allow, that this technique made a difference in how much gas actually got into my tank. Since gasoline has a much higher evaporation rate, I'd actually think that pumping faster and then capping the tank more quickly would keep more 'liquid' gas in the tank. His reasoning was that if you pump it quickly, it causes more air bubbles, so you're paying for more air.

Which is it? Does it even matter?

It doesn't matter.

Have you considered that some fuel delivery systems have a vapor recovery sub-system? Could filling faster cause more vapor to be created and then sucked back to the stations recovery system and out of your pocketbook?