As far as space is concerned, think of the sportswagon as a hatchback rather than an estate. The boot is smaller, I believe, with the seats up than the saloon. But you do have more flexibility.
As far as engines go - I have owned a 2.4 20v 156 and had 2 Alfa's, a Vauxhall and a Fiat with the 1.9 engine in it.
I don't care if you can get the 1.9 re-mapped to give as much power, all you need to do is drive a 2.4 and put your foot down. I have never heard a diesel sound that good whereas the 1.9 sounds like - well, a diesel!
The 156 20v was my favourite ever car - and I've had a few! If you can get a well cared for one, go for it.