I have had a GTV, Spider and 156
The 156 has the turning circle of the GTV, and that means the responsiveness as well, given that it is a family saloon it does handle well and with far more comfort than a GTV.
Common engine issues are similar to a GTV of you are looking at getting a twin spark powered 156, the wishbones sometimes give issues as do certain bushes on the back suspension.
The driving position in some ways is nicer than the GTV, more relaxed, but the dash layout is similar and that's what makes jumping from a 916 car to a 156 so easy.
I could easily get 35 mpg on a run with my 156 2.0 selespeed and about 25 mpg around town as well....
There does seem to be issues with floorpans on the 156, they seem to be prone to rust so that's one are worth looking at when buying