Typically the 1.6 should get between 33-35mpg
2.0 should get 30-33mpg
1.9 8v should get 45mpg+
1.9 16v should get around the 40mpg+
Diesels are usually cheaper when you pay at the pump. Little bit more on servicing.
If you dont do that many miles or short journeys or around town get a petrol. If you do long distance motorway miles get a diesel. Its that simple really unless you have to have a specific engine type you prefere driving. Then that will over rule it.
If I was to get the diesel though although the 8v is great, I'd opt for the extra valves. Unless the 8v was a bargain in which case my first port of call would be to get it remapped.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts