This really is a question of personal preference. I have a 166, and am hugely biased towards it. I think it is an absolutely wonderful car.
Personally, I have a huge problem with the 156 - I cannot stand the interior. I adore the 166, inside and out. Now, I am happy to admit that I am a little strange in this regard
but I much prefer the 166 as a car.
The GTA, on paper, looks an absolutely awesome machine. You can get a 6 speed selespeed box on it, and still have the V6 and 250 horsepower. Unfortunately to me, the deep bumpers and the sideskirts look far too Halfords for my liking.
Saying that, I was at Castle Combe over the weekend, and Alfaholics were having a track day. They were taking people out in a GTA saloon, and running 1:32 a lap - three people in it. I thought that this was very quick.
Price wise, I would have thought that the 166 was far cheaper, both new and used. Those GTAs seem to be in real demand.
I would say go for the 166.
ps: To all 156 owners, and particularly GTA, please do not napalm my house after reading this....