I went for a BMW, and it's petrol, and its got a lot less horses than the Alfa.
But it drives soooo much better, that the lack of torque or power is compensated for 10 fold by the quality of the thing.
The BMW might be sparser/more compact with regards to interior. But, it's nice and functional. The leather is much nicer than that hard, old man type stuff in the 159. The seats are far more comfier (I just could not get a good driving position after constantly attempting to adjust the seat in the Alfa).
Make sure you have a proper go in the 159 you plan to buy first. I bought it after reading all the good stuff, and wished I had taken more time to test drive it first. The fancy dashboard gets old really quick after you've gotten used to it. And the 159 seems terribly small inside. Perhaps that's just an illusion, but it seemed more cramped than my missus peugeot 307.
I took a week off work not long ago, and the 159 was that much of a chore to drive, we reverted to driving her 1.6 peugeot all week!
My 159 had 2 major problems:
1) It totally crapped out after 2 weeks of owning it. I think it was the EGR that went wrong, but they replaced all sorts of other stuff too.
2) The hand break (all though put on tightly) didn't hold it and it ran into the wall of my house - £1,000 for new front bumper.
Nothing else actually went wrong, but it always felt like something was just about to go wrong. Every time I drove it, I expected it to conk out.
Good luck if you buy one. I hope it just was mine that was particularly crummy.