The 2.2 & 3.2 differ little in reality. In my 2.2 I used to average 23 but could get 31.
In my 3.2, I average 19 but can get 28.
In some ways, the 2.2 was more fun as you have to thrash it to get it going but it loved that
With the 3.2, you can laze around at in 6th at 20mph or spank it to 77 in second.
As for road tax. £490 plays £270 so that's nothing over a year.
My biggest regret of Alfa ownership is not buying a new 3.2Q4 Ti 159 when the first came out. Had one V6 already so played it safe with a 2.2.
Had I bought the 3.2 159, I'd still have it & wouldn't have got either Brera.
3.2 Q4 159 Ti is, IMHO, the best Alfa ever made aside from the 8c & forthcoming 4c.