Sent you a PM on FCCUK but I'll post it here too.
Standard the 1.9 is not bad, its not the quickest thing about but its respectable, with regards to economy I don't think there is much difference between the 2.4(175bhp, 20v) and the 1.9 (150, 16v) on average I'll do anywhere between 39-44mpg over a whole tank, mine is remapped and I do use the power a lot rather than drive for economy, typical range from the 59-60 litre fill up is 530-570 miles, if I drive with a light foot I have had that right up to 650 but thats only happened once or twice. If you drive a lot in stop start traffic never really getting anywhere I would expect 36-39mpg.
For me to be happy with the 16v I had to have the remap, once that was done I got most of the sensation of speed the GTA had bar the lovely noise and the shocking fuel consumption, I've had my car so long because I really do enjoy it, I even like how it sounds (its grown on me) standard the GT is pretty comfortable and able its softer than the V6 with a bit more body roll but its not terrible, I would uprate handling on any Fiat/alfa as they don't suit my taste, all the diesels lack proper sports suspension as Alfa don't market it as a sports car.
On the remap side of things the usual suspects can sort you out, I'm currently running and Angel map and its very good(177) though I did have a better map from Celtic tuning ( 190, they claimed a larger bump than I got so I had my car reverted until I do some supplemental mods at which point I'll get it tuned again, want to crack 220) *
I'd go for a GT whilst the 156 has the better engine (2.4) its not the better car, the 2.4 is a cracking engine though with a great sound, I don't think the 156 is as practical as the GT, the rear hatch back and fold down seats make it pretty handy.
You can ask any of the guys on the fccuk forum who came to North Weald if my oil burner is any good, it battered most of the Coops, probably came top 6 out of 25 or something like that on the track and the slalom even with ****e tyres
bare in mind that the top 3 were F430, Chim 500 and the caterham its not too bad, when you look at power to weight the car pulled a blinder
* my figures are from back to back Dyno testing of the remaps at Powerstation and not numbers thrown in the air like most people.