The first difference that I can remember straight away is that Alfa Romeo 156/147 GTA use their ABS sensors for the speedometer reading.
Fiat Coupe uses some kind of plastic gearwheel attached to the diff. So there are 2 notches/holes for this gearwheel to be locked. I think there aren't such holes in the GTA diff because they don't need it.
Although this is not a major problem if you use the car only for track days...
You can use Alfa 156/GT Q2 diff without a problem, it fits right into 6 and 5 speed diffbox as well. You need to purchase a gear speed sensor which needs to be put on the differential OE PN fiat 7783658.