The GTA is much more focussed machine than the GT it has a much sharper chassis set up with even faster steering than the GT, the rack on the GTA's is the quickest on any production car at 1.75 turns lock to lock.
The seats on the GTA are generally bigger in fact they are enormous!, if you are slim you will fit in a GT OK but if you are of ample proportions you might get pinched a bit
The GT might have the edge on looks but this is subjective. It's also still a current car and probably will be for a year or two. The 156 isn't sadly.
If you need four seats and four doors the GTA is the obvious, if you just need two with occasional use of the back then a GT would be fine. Practicality wise the GTA Sportwagon would be the best.
Both are fine cars, very fas and reasonably refined, the GT probably has the edge here and might be a better cruiser, though I have found the GTA pretty damn good at that too.