Brera v GT, lots to consider most of which has already been covered but, a few thoughts for you:
Brera is a more modern design, much safer and has a stronger shell in the unfortunate event of your Hanching Ditchfinders losing their grip.
GT has superior rear seats, the Brera's are made for kids and Douglas Bader though, normal sized adults will fit in at a push for short journeys. The rear of a GT is a dark place, the Brera with SV has more light and feels less claustrophobic inside. Having said that, I have no issues with the inside of my Brera S but, I came to it from a 159ti with its black interior and roof lining!
Get the standard 2.2 mod'd and it will be producing near as damn it 200bhp and can easily return averages per tank of around 30+mpg on normal driving with a nice sound track and a very linear power delivery which does not feel that fast but, should be nearly on a par with an unmoded 2.4. The 2.4 of course dumps its power hard into a narrow band and so you get the kick in the back, change up, kick in the back, change up etc while the 2.2 is still in the same gear and working it's way up the through revs and keeping pace. Expect low 30's mpg from the 2.4. You get the same power delivery from a 1.9GT but just not so viscious while returning 40mpg.
Certainly with the 2.4 expect issues with EGR and or DPF at some point although the 1.9 is not averse to these problems. Out of warranty these will not be cheap. My 2.4 159ti had issues around 18 months old which under warranty cost over £1200 to rectify. Personally I would not buy a 2.4 out of warranty or, if I did, I would get the DPF removed which opens up the power, increases mpg and makes it more reliable, costs I believe are in the region of £600 - £800.
As for handling, the 2.4 is a nice cruiser which handles well enough but, it is ultimately nose heavy and set up for comfort, it's a GT car really. Fine for most people and the unitiated but, if you get the chance to drive a Brera S you will realise how badly Alfa missed a trick with the Brera and what it could have been especially with the steering feel and handling. The ride is firm not hard and handling is in another league compared to a standard Brera. A 2.2 S straight out of the box will outcorner many Q4 cars in the dry as well as any model of 156 or GT. Fact.
So, it's all about what you want out of the car. My 2.2 S is the best Alfa I have ever owned, nothing has gone wrong as opposed to my 2.4ti, it looks drop dead gorgeous, I regularly get people comment on it, when we have club meets it's the car people tend to gravitate to and end up stood around mostly because it's what most Alfisti like and aspire to in the way of a newer car. If I was going to do lots of motorway miles then the 1.9GT makes sense. If I was going to do some motorway and regular long journeys I would get a Brera 2.4 and get it mod'd with DPF removal. But for now, my Brera S 2.2 is the dogs dangly bits, the lightest Brera ever made, the rear Janspeed boxes loosen up a few more horses and give a glorious sound, the Mangos's Mod4 brings all this up to around 200bhp and gives a nice induction sound, and it fly's down the road hanging on the back of cars with a 0-62mph time of around 8.0 seconds but, then it is so much lighter than early series models.
I know where my money would go
Look on these forums and see how many issues people have with the 2.2 engine, almost none. Now do the same with the 1.9 and 2.4 diesels. Hmmm.