what a nice dilemma to have !
well the first question you have to ask yourself is what are the most important characteristics to you in your new drive....
I have recently been in a similar situation and bought a GT 1.9 JTDM blackline, which consequently has had a good bit of modification but I digress, here are my experiences and thoughts from driving 3 brera's 2 159's and several GT's, I am not a family man but as a pro guitarist boot space and the occasional passenger duty is important.
oh and the brera is not out of your price range, you can pick one up for around 5k now, even cheaper in fact and if the history is good and the necessary work has been done these can be a good buy.
ok so the brera first...
I drove 3 or 4 of these with both petrol and diesel engines and found the 2.2 petrol under powered for the weighty chassis of the brera, in addition the I had had problems with a JTS in my previous GT and wouldn't buy one again the 3.2 is thirsty!. The 2.4 is a great engine and sounds OK for a derv and has ample power, However the economy is poor for a derv again due to the weight of the chassis, a remap would help but be prepared to replace the clutch in short order if you do it, Vs the GT the drive is so so IMO the weight meaning it is not hugely nimble or chuckable. the build quality is good and its free of some of the rattles of the GT, it feels more solidly bolted together,is a very capable motorway cruiser and looks stunning!.
Inside is lovely, the SV roof mkaing it feel more spacious than it really is, boot space is ok but access is not great as its a funny shape, rear seats are only usable for little ones. Teenagers may well complain at the lack of space.
the 159 is the same in drive-ability as the brera, they share a chassis but it obviously has better rear seats and access to the boot, with a family I would choose this over the Brera.
now onto the GT, handling wise it trumps the Brera and 159 hands down as its lighter, more nimble and feels like an alfa, (sorry Brera and 159 owners but I don't think they have the ''alfa magic'' handling wise)
that said the stock suspension is roly poly and if you really want to enjoy as a sports coupe it you will have to make some changes, for family use however its fine in stock form.
the trade off for the lighter build is that suspension components are almost a service item needing new upper wishbones every 30k or so and lowers around every 50-75k...the rust proofing is not great on some from the factory so I would budget for this given our climate, economy is far better though and I average 50-55mpg no problem.
the GT has a slightly more dated interior it's still lovely but I have to say the brera wins this, the GT does however have massive boot space in comparison and more generous rear seats so wins for me on family use,I still love the look of mine and its comfortable and more roomy than it you expect.
the 1.9 JTDM is a great engine and will do serious miles without complaint, EGR delete is a really good idea and again I would re-map for maximum enjoyment and fuel economy as well as doing the swirl flap delete mod if you are buying a later CF4 engine...earlier CF3 models can be had for peanuts and if i had my time again I would buy cheap and keep funds aside for the suspension ,cambelt some mods and any unexpected's.
I think the GT is a good midpoint between the Brera and 159 on most aspects offering better economy than both and more boot space and interior room than the brera. IMO the GT is a stunning car to look at and my lowered one looks as good as any brera, added to which you can tell the front from the back without looking twice..!
if I were in your position of having to regularly transport kids I would actually probably go for neither and buy a facelift 156, best of both worlds, closer to the 159 styling but with the light nimble chassis of the 147, and GT, 4 doors for the family and nice light drive-able chassis for you!