I'm a member of another Alfa owning forum and chatted with one of the more senior guys on there in some length when purchasing my 159. He had one of the original bells and whistles TIs when the 159 was first released. His was the 2.4 and the MPG he was getting was pretty pants, around town he was experiencing low 30s for MPG, at which point you'd have probably been as financially well off with a petrol with differences in fuel price etc.
The 2.4 is nice and smooth and makes a nice noise though, albeit the 1.9 feels heavy enough already compared to the 156 before it and the 2.4 is a beast of a motor.
Depends on your preference, if you're not that bothered about MPG and want a nice smooth ride to bumble along with then go for the 2.4. If MPG is more of a factor and you do a bit more miles wise then I'd go with the 2.0 - they're a decent enough engine and will do the car justice alright.