I had a 2000 - 156 2.4 (10v-136 bhp) SW with Sport pack owned it for about 4 yrs before finally selling it with 250km's on the clock. I replaced it with a 2008 -159 Ti 2.4 Q4 SW (210bhp) with only 70K km's on the clock.
Of course these are two completely different beasts and the 159 cost me 7x what I sold the 156 so the comparison has to be put in context.
In alot of ways the 156 was more fun to drive but alot of this I think is down to the speed's involved.
You could push the 156 harder and rev it a bit more because your not going so quickly.
Yes the steering was definately more responsive in the 156 than the 159.
Room for the rear passengers is the same....Limited.
The frount is probably a little bigger in the 159 in the passenger compartment but not as big a difference as the external dimensions would suggest. The 159 is alot bigger car than the 156 and most of the size seems to be taken up with crumple zones and air bags so I'm sure it is alot safer car if you do have a shunt.
The Boot of the 159 is definatelly bigger although my personal criticism is that the accountants at Fiat did away with the intergrated dog guard / load mesh/ load cover...Which IMO was a really great feature on the 156.
As other people have said the 159 is more like a GT car than the 156...it gobbles up motorway miles but still manages to put a huge smile on your face in the process and on the twisty stuff. The grip of the Q4 Ti is epic.
Regarding servicing...I used to do everything on the 156, I haven't actually started on the 159 yet but have bought a full set of filters etc to do it next time. What I can say is the price of the filters alone is alot more than those on the 156 for the same brand from the same supplier.
Within 3K km of buying my 159 the EGR valve blocked and the DPF was destroyed as a result leaving me with a 3000Euro bill as I have no local indi to by pass re map. Fortunatelly it was partly under insurance warrenty and the BMW dealer I bought it from paid part of the bill so I accually ended up paying 1/3rd.
The suspension is as flaky on the 159 as was on the 156....but the tyres especially 19" ones are about 3x the price of the 16" on the 156. I've used a full set in 25k Km so booked the 159 into Autolusso for the powerflex conversion, on the basis that the saving on tyres will pay for the upgrade in a year assuming it doesn't scrub another set in a year!
With all that said would I go back to a 156 ?
The truth is the 159 Ti 2.4 Q4 is a fantastic bit of kit, it puts a smile on my face every time I drive it.
and because its an Alfa you forgive its weaknesses.
If you do go down the 159 route, get the EGR Blocked, DPF deleted and Suspension bushes done then IMO I think you will have a reliable car that you will love more than your 156..