I think that the difference in fuel mileage should be at the very bottom of your list of concerns. The V8 car is a different animal, even though they share the same chassis series. I fully expect that they are good cars but there are several things you should consider.

The engine compartment is much more cramped and difficult to work on. Even if you don't do work yourself, you will have to pay more for most all repair operations. The timing chain and components need more attention and you are just dealing with a more complex engine.

It stands to reason that in the long haul this will be a more expensive car to operate regardless of any difference in fuel mileage either way.

All that said, if the added expense is not a problem for you, the V8 car should be a real joy to drive.

Good luck,