Hi TimboZero - have recently gone through the same dilema as you! Brera vs. 159. As you can see from my avatar - 159 LE (1.9) was the final choice. Ill take you through my train of thought;
First thing to consider was trim. Not just internal but also external. With the Alfa internal is a little less important - because apart from Turismo, the others have plenty of toys and leather as std.
External trim, therefore; was a key deciding issue. I was after big rims, low profile tyres, lower ride and harder suspension. For the 159 - the TI was the obvious choice, whilst with the Brera it was the S.... (Prodrive model)
As the Brera S was considerably more expensive than a 1.9 TI, I decided on the 159. However, I was still unsure about some aspects of the trim on the TI. The silver side mirrors for one, and the red stiching on the internal leather for two.... but I thought - hey ho... cant get everything in life....
Lucky for me - a couple of weeks before I was due to put in my final order - the Limited Edition 159 was launched.... and guess what - it comes with a near trim to the TI as possible without any of the gimmicks like side skirts, silver side mirrors and red stiching!! Even better - darkened TI 19s!!!!! Decision was done!
As a foot note - i decided on the diesel for economy reasons......
The 1.9JTDm decision was based on the fact that after re-map (which im doing on the 9th May) - power is very near 2.4jtdm. Difference though is that the 1.9 unit is lighter than 2.4!!!!! Therefore, less tyre wear and acceleration should also be similar to bigger engine, following the remap.
In summary - go for what tickles your fancy!