I have the 1.9 Ti.
http://www.flickr.com/photos/gregkingston/6886708704/
Alfa 159 Ti by
Greg Kingston, on Flickr
It has been remapped, had the DPF removed and EGR bypassed. Without this work doing it would be a severe disappointment. The 159 feels heavy, especially on 19" wheels, and the engine in the 1.9 at stock is just simply underpowered. And, as others have said, there's no payback in economy either.
Once mapped etc (the work cost me around £750 but other versions are available for about half) the car is transformed. Much more freedom in revving, and probably just gets the nod over a stock 2.4. The economy of the 2.4 is even worse of course, but once remapped it does have a lot more power. My 1.9 sits at just over 200 bhp, the 2.4 can go up to 250+.
My long term mpg since the mapping is 44 - about 6 or 7 better than before. I cannot emphasise enough however that I would not have kept this car without the remap, despite the beautiful looks, interior and exterior.
Ti is the one to go for though, if you don't mind the bumpier ride on the 19" wheels.
A few more pics here
http://www.flickr.com/photos/gregkingston/sets/72157627470215830/