Because I'm a saddo, I keep records of MPG.
My old five speed Coupe:
Over 5892 Miles
GTV (6 Speed)
Over 5533 Miles
A coupé with the Six speed box would easily return 32MPG + at a 80+ cruise on the motorway I reckon, if not more.
Which one is the best everyday car is dependant upon how you use it? If you do a lot of motorways I'd be tempted to say the Fiat is the better choice, if it's more A/B Roads then I'd say the GTV as it's more fun. If its purely down to MPG however I'd say the Coupé is the better bet. My figures above are somewhat skewed in that I mainly use the GTV on Motorways when I drive it to work, the Fiat however was my only car at one time so it was used for everything.
EDIT: The 21MPG minimum I got with the Fiat was when it had a leak so it's not valid really. It was incredibly hard to get below 25mpg unless you drove with a completely binary throttle, the GTV on the other hand is quite hard to get above 25mpg imho.