Kind of a matter of taste. '66's were more "slab-sided" where '67's are a bit more rounded.
The 67 being 2 inches wider and longer made for a smidge more room inside and a smidge more room for bigger V8 engines under the hood.
Old, only option for AC was a gawky looking box thing hanging under the dash. Starting in '67 regular "in-dash" AC was available.
And I personally like the optional brushed aluminum "Deluxe" interior trim you could have (or add). Some people like the 66's "Pony" interior option better.
Another '67+ option wasa the "tilt-away" steering column. "Late" (but fairly rare) 67's got collapsible steering columns which are more crashworthy than early 67's and all 66's. It's not hard to swap a collapsible 68 column into a 67 (I did it) but a whole lot more work to get such a feature into a 66.
I suggest picking the one that makes you feel good every time you look at it, even if it happens to be broken at the moment. That'd be the right one.