It depends on what your going to use it for.
If you are just going to cruse it than which ever you like better. If you are going to beat it,than the 66 would be the better choice. The 64 has a 2 piece drive shaft and is the weak link and very troublesome. The 66 has a 1 piece drive shaft and is a lighter car.
Good question, I have owned both cars , both have been ss models.I would have to say the 64 was a great car but my personal opinion is I would go for the 66, myself I enjoyed the look of the body and the power of the 396 engine. The 64 body is a bit to boxey for me. But hey to each his own.