I have owned a 1999 156 2.4 JTD and 2005 156 2.0 JTS before, currently own a 1.9 GT JTD and 3.0 V6 GTV. So got a fair bit of experience behind me
Random points as they come to mind :-
156 2.4 JTD has greater pull/sound than the 1.9 unit.
Alfa GT feels tighter and more agile
Fold down rear seats in the Alfa GT are great, the 156's are fixed!
Space for kids in the back of the GT, but ultimately the 156 is going to be better for this.
2.4 JTD engine is quite heavy so does induce greater understeer
Remap on a 1.9JTD will also bring about 180-190 bhp
PS: Are you the Lee that called me about my GT a few weeks ago?
If so, I am also on this forum