the 3.2 159 does not drive like a 3.2 GT, but remains an excellent vehicle IMO. i have both.
the GT feels more raw, with a higher degree of vibration, 'feel' through the steering wheel, harder suspension, a very menacing engine sound, and a huge shove from acceleration. it is trickier to drive fast, but all the more rewarding.
the 159 is more solid and refined, so it masks most of the above. it is faster than it 'feels', and the level of grip from the Q4 is immense. the acceleration is more linear, rather than a shove from a brisk rise in power/torque midway along the rev range. my car is a Qtronic, and i agree with 166lover and Kalel that it is a great transmission. the only minor quibble is that it does shift gears when near/at the redline rather than 'hold', even in sequential (paddleshift) mode; that's the nature of an auto i suppose.
i think, with the right basic mods, a 3.2 Q4 'manual' could be as quick as a 156GTA, with higher levels of grip and safety. exhaust (high flow cats, new centre pipe and backboxes), intake, and remap, and if you're hardcore add coilover suspension, and it should easily be a sub-7 second car to 62mph.