Ooooh, design by committee, cool
I prefer the Pontiac, and it happens to be my fav year GTO. I like the one year only details on this car. Having said that, as mentioned already, it'd likely be cheaper to build the Buick as these are generally less popular than a GTO so prices should be less and more parts are potentially available with the 1971 model and 1970 model years than you could swap from a 69 to a 68 Pontiac. I still see 70-72 2 door buick skylarks pop up on craigslist out here for 1-2k. They are overlooked cars, imo.
You won't go wrong with either car, really, and you'll stand out nicely against a sea of Chevelles.
I did have a '72 Buick Skylark while in college. It was fun to fishtail down the snowy roads in the cold Cleveland and upstate NY winters. It was the best $250 I ever spent for a car. (it was a 4 door HT and this was the early 90's)