I prefer the '67 for both styling and engineering. All of the reasons I feel that way have been mentioned, so I won't recycle them. But I will add locking seat backs on the '67 to that list.
The only body feature of the '66 I do like is the tail light design, but they honestly were hard to see in the daytime. I never saw them at night, but plenty of '66 owners saw my '67 tail lights.
In the early 70's I did lots of night time street racing. Of the hand full of '66 GTOs that I ran against, I never lost. My brother had a '67 4 speed that was just as fast.