in addition, i'm going to say something controversial now...
there are guys with even older Alfa's that may disagree with this opinion but I think the Brera and the 159 represent a significant change in ethos from Alfa ,with the two models being heavily focused on build quality and reliability rather than the usual priorities of fun, handling and being a bit different.
it seems they sacrificed some magic to achieve this with this platform, being heavy and cumbersome they will never be the drivers choice of car, but they ARE beautiful to look at and as a family car the 159 is hard to beat with its sumptuous trim levels and roomy interior...the 2.4 derv is also a good compromise with better economy than the smaller diesel engine in this car and the 5 pot gives a nice sound for a derv!
but in engineering there is always a trade off.
my point is in years to come we may well look back and see the 159/brera as a clear line in the sand between New Alfa and old Alfa.... so I suppose its a case of making your choice based on your needs, the 156 is more fragile, no doubt but IMO the design and build ethos remains largely Italian where as the 159 has a more Germanic ideology at its core.
we can only hope that future models will manage to balance the two rather contradictory imperatives..judging by the look and Specs of the Guilia things are looking good on that front, I just hope Alfa don't perfect the magic out of the new cars in order to chase market share. that would be a shame.
I own many things whose imperfections make them what they are and I consider my GT to be of that ilk.... so obviously for me it would be a 156 all day long, I drove several Brera's before buying my second GT and I really wanted to love the car, they look so beautiful and the sv roof is such a good feature, but with a 30k a year mileage I needed something to make it less boring from time to time, so handling was important, as was economy...for me the 159 platform does not have ''it''
it has lots of things the 156/147/GT platform doesn't for sure, only you will know which characteristics are the most important to you.
good luck with your purchase