Haven't had a 2.4 156 but I have had a 159 2.4 JTDM and now have a 156 V6. Personal opinion but the 159 is quite simply not as nice a car to drive; it feels much bigger almost boat like and it doesn't give you the same excitement as a 156. In a 156 I feel like there is an element of control and interaction whereas in the 159 I felt like a passenger despite all of the upgrades/alterations (DPF cored/EGR delete/ more BHP etc.). The steering wheel is quite small and the front end looks and feels heavy from the drivers seat.
That being said, the 159 is a great looker especially the the TI guise SW. I still don't think there is a better looking car on the road. It took me a lot longer to warm to the looks of a 156 but I appreciate it so much more now I have one. I suppose it depends on what motoring is to you really. Is it getting you to and from a location or is it about enjoying the drive of a car. If the latter, the 159 doesn't feature for me but again, I am contrasting a 156 V6 against a 159 2.4 JTDM. If I wanted someone to tell me how nice my car looked everywhere I went, I'd go with the 159.
Although fairly new to Alfa's, I have a 156 but was loaned a 159 last week when my cambelt was being replaced. I can't really talk about the various engine options but have reflected on how the cars made me feel. To my eye the 159 looks superb and seemed fairly well screwed together but lacked a bit of personality. Having owned several eurocars, e.g, BMW, Mercedes, Audi, Volvo and Saab's as business cars over the years, didn't think that the 159 felt much different, efficient and admirable but not loveable. My facelift 156 however is fun, is nimble and somehow feels like I expected an Alfa Romeo to be. I think we're going to have a long relationship.