Just to add my penneth worth of opinion on the original post topic of petrol vs diesel... I took a deep breath of hesitation when deciding to purchase a 159 Ti 2.4 JTDM rather than the 2.2 JTS (as was then the main petrol option). It has been a mixed bag of experience.
On the positive side, it is a quick car and noticeably good at mid-range overtaking acceleration when on the move. Also, for a diesel, once it's warmed up it is pretty smooth and you'll doubtfully remember it's a diesel.
On the nagative side, personally I just can't get that same sense of 'bonding' or passion with this car that I have had with my previous 6 petrol Alfas over the last 20 years. When you start the car up in the morning you get that nagging feeling that perhaps you made the wrong choice. You are just hoping there's nobody walking past at that moment to hear the bag of nails that greets the diesel driver. And don't kid yourself that this diesel sounds better than all other diesels. It doesn't. But equally whenever you drive the car around town where people are more likely to notice the car's shape than the speed, you do feel this un-nerving sense of embarrassment especially as you pull away from traffic lights fearing the titters of "oh no, it's a diesel". And don't even think about racing away from the lights, it's quick but it judders terribly at near wheel-spin acceleration. Then there is the sheer weight of the car. The 2.4 diesel is nearly 1.7 tons! I admit that despite the weight it is a very good ride (much more sure-footed than my 156's or 146) but you ARE aware that you are driving a heavy car. I don't actually know that the 2.2 JTS feels lighter, but I presume it must.
I would say that the right engine depends whether you are (or want to be) a round-town poser/boy racer (go for the petrol because it won't embarrass you) or a motorway motorhead (go for the diesel as it's great at acceleration and the MPGs are better... just).