I've had 3 156's, 3 147's and a GT - all petrol. Let's be honest here. The platform that came first was the 156 saloon. This was extended beyond the rear wheels for the sportwagen. The 156 saloon platform was shortened by 5cm for the 147. The GT is based on the sportwagen chassis and is generally considered to be the best sorted, in original spec, of all the 156 based saloons. Working on this lineage the GT does not sit on a lengthened 147 chassis.
I'd say you'll feel more special in a GT than 147, but go for as big an engine as you can get. If that means 170bhp diesel then so be it. 3.2 petrol would be nice though.
GT's are suprisingly practical, given that they are a coupe. The boots big and more useable than a 147's. The rear seats are a little less accessible than the 147's due to the lower roofline, but the seats are just so comfortable provided you fit in them. Unfortunate I have a friend who finds them uncomfortable because he only fits on them.
Watch how much you put in a GT, it weighs about 1300kg and max's out at 1800kg. I think it's a bit more in a 147. You can't fit a roof rack to a GT, a boot mounted bike rack shouldn't be fitted to either if it clamps onto the glass.
If you've never had an Alfa before whichever you have you'll love. If you buy the 147 and then have a GT, you'll realise what people are writing about on this thread, if you don't you'll only realise when you've sold it.
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........