Hate to be the bearer of bad news mate, but I'm almost certain that the Alfa will cost more to run, with that big 3.2 V6 I doubt you'll return anything more than 25MPG when using her daily.
On a run mind you'll probably hit 30MPG.
Also, and don't hold me to this, but I think the GT was one of the last Alfa's to use the Arese V6, as opposed to the new GM V6. Meaning that every 72,000 miles / 5 years it'll need a cambelt change. So if you are looking at a car that's nearly 5 years old or coming up to it's big Seven Oh, then factor in the cost of a cambelt change which is about £500+ including water pump, tensioners etc.
That said mate, a few hundred quid every 5 years isn't that bad, but if it soon needs a belt change do remember to factor that in, because alot of the cheaper GT's will be coming up to their fifth birthday soon.