Having driven both (including a 155Q4 and 156 GTA), it is a complex matrix based on which version and condition these days. Personally I feel that the 155 V6 is utterly lovely and great if you do not require max progress. It sounds great, handles well (if in good nick), and can be easy to maintain if you are handy or have an Alfa mechanic nearby.
The 156 is a massive step up in design. However it uses even more plastics and sensors. And the electronics are both a boon and curse. But this is with hindsight. At the time this was a ground breaking car not just for Alfa, but all smaller production manufacturers.
Have you seen one in person? There are a lot of details that differentiate it from a Corolla.
Handling is not your usual FWD either.
TG review