I don't think it's quite as simple as that
The 1.9 JTD 16v 150 is strong through the gears, the torque is prodigious in third and fourth gears, matched against the 3.2 V6 in the wrong gear and the diesel will destroy the V6.
In any case, when did Alfa Romeo ever build logic into their range?
Seriously though, you need to decide which model fits your budget. The V6 is a landmark engine, perhaps the finest V6 ever made, as a package the GT V6 is essentially a junior Ferrari. You will pay for it though in fuel costs, insurance, and service costs. The JTD 16v again is an excellent engine of its type, not in the same league as the current diesels produced by BMW, but will give good service and fuel consumption if serviced correctly.
Go and test drive both and see what you think.
Either way if you enter Alfa ownership and leave the Vauxhall behind, you will be entering into another world of sensory perception. Generally a much more pleasant one, the memories of your previous motoring experience will quickly fade, as the human brain is clever at remembering the good things (i.e. pleasure) and is quick to forget the unpleasant (i.e. pain).