twincamms: more specifically, what engined GT, to what engined 159?
in terms of chassis and refinement, the 159 is superior. easily. just a lovely, relaxed car to drive. nothing new there.
but i'd only do the change if you have a similarly powerful 159. i wouldn't change from a V6 GT to a 2.2 JTS 159, for example.
things like styling, size, practicality, etc, are all subjective. so you will have your own opinions on this; and others will have their own opinions (which may not be valid to you).
if you're looking for a car to upgrade to, then i think the 159 is an excellent choice. though if you wish to retain having a 2-door, the Brera is probably a better choice.
for a general, all round vehicle, if i had to choose between my 3.2 GT and 3.2 159 Ti, i'd take the 159. it's just better in so many ways. of course, that's subjective too!