It's simple. If practicality and fuel cost is an issue you need the diesel. If you want a car as a keeper that you will love and others will admire, then it's the V6. The Busso V6 is the heart and soul of what Alfa is about, and to have that in a lovely car like a GT. Just run through these scenarios:
Joe public walks up to you and the car. "Nice looking car, what is it?" An Alfa GT. "Nice, what engine has it got?" A diesel. "Oh." Joe public walks off.
Joe public walks up to you and the car. "Nice looking car, what is it?" An Alfa GT. "Nice, what engine has it got?" A 3.2 V6. "Wow, I bet that's nice, what power does it have." Cue long discussion about bhp, what Clarkson says about Alfas, how they wish they'd had one or someone they knew had one and loved it etc etc. You can even lift up the bonnet and have them go "wow" again.
Of course, IMO you will feel the same way yourself as Joe Public in scenario 2 if you have a V6. I am driving a loaner GT diesel, and I can say that it does not stir me the same way the V6 does. It may be practical but after years of driving twin sparks and V6s, starting a diesel is a bit of a let down.
But if you do go diesel for practicality, at least it will look nice from the outside.