Both are very good cars maybe though inslightly different ways.
I had a 3.2 Brera for a couple of years, absolutely lovely & as reliable as any other car, really. I changed to a 1.9 GT for practical reasons, as well as mpg reasons. After 2+ years, the GT has been equally reliable. It presently has approx 44k miles on it.
The Brera definitely has a more modern feel, it's a step up in build quality & feels as solid as any BM or Audi I've been in. However, although the Gt feels & looks older inside, I think it still works well, everything is handy to reach & it's a very comfy place to be. I think the front seats in the GT are better than the Brera's, or at least the early Brera at least. The GT has much better rear seats & is easily the more practical car. I don't really mind the GT's interior being dated. One mans "old/dated" is another mans " future classic".
Actually I am at present thinking of going back to a Brera as I no longer need the rear seats practicality of the GT, although it would have to be a 2.2,not the 3.2 sadly.
So summing up, both lovely cars but in slightly different ways. My experience is that both are equally reliable, the depreciation damage on both has largely been done. If you don't need the extra room & practicality the GT gives & don't mind the poorer mpg the Brera gives, I think I'd go Brera.