Fuel prices, especially here in Italy, are very high. However, the differences in fuel efficiency and/or fuel type for the various engine options isn't all that great. NONE of the Alfas are known for sipping fuel.
Each engine option has it's pros and cons but none of them are horrible. When I went shopping for my Spider I decided that any of the engines would be OK both from a fuel efficiency and a performance/fun point of view. My thinking was that finding the best used car was far more important than shopping for my favorite engine and then accepting whatever car came with it.
The car I found was always garage kept, had low mileage, always maintained by the dealer, had new tires, zero scratches, it was the color I wanted, the interior looked brand new, etc. It just happened to have a 2.2 JTS.
Facing any kind of complicated repair on a Spider or Brera is going to be MUCH more shocking than the disappointment you will face when calculating your fuel mileage.
My advice is to hunt for a great car, not the most fuel efficient engine.