The 66 and 67 are so close when you just look them over. I spent a few months deciding which I preferred. For me it came down to the rear of the car. I LOVE the 66 louvered tail lights but the bumper and q-panel trim flare out in a large crescent for use with the Tempest/Lemans tail lights. It just doesn't flow for me. The 67 has the straight bumper and trim line to follow the tail lights. I wish Pontiac had kept the 66 lights as well....

I actually prefer the 66 grilles a little but, again, they are so close to the 67 that it isn't an aesthetic issue for me. The interiors are almost identical. There are subtle changes as with most cars of the day to keep buyers coming back for the "new" model. The one thing that surprised and cost me a chunk of change is the fact that a number of the 67 components are a 1 year only and can be hard to find and expensive when starting with a stripped, rotten car like I did. The rocker trim is massive and hard to find in decent condition at a cost that doesn't require a second mortgage. The steering column and pedals can be difficult to find. The frame has a slight change at the front crossmember too and I drove from MN to central OK last spring to get a correct frame for my car. My car will by no means be even close to numbers matching but I do want to retain as much of the original appearance as possible. I don't care for Black interior so that will be the biggest change from original. It will be finished in Parchment and Dark Turquoise.
You're tastes will dictate what options you want like either an automatic or manual trans, A/C, PS,PB, convertible...etc. Now is a good time to buy with the prices still depressed but take your time and do plenty of research on how to decode the trim plate to check for originality. Also, once you get serious about a particular car, get the PHS documentation for it, if the seller doesn't already have it. It will tell you EXACTLY how the car was configured at the factory. If looking at a mid to high end restoration by all means have it evaluated by someone with experience in these cars. Rust issues alone can be a big problem later, if they weren't properly addressed.