One of the major factors that impacts resale value today is the market for restoration parts. You can build a complete '69 out of a catalog. You cannot do that with a '70. Easily factor x 10 harder to do a correct '70 resto. That greatly impacts current value in the resale market.
Second generation cars were also very old in the tooth by the time they got rid of them in 81' they were well past the muscle car period.
Which car is better? All you have to do is read the reviews from the period. They tell the story.
BTW.. I also own two first generation cars so I like them too,and believe me they are lots more fun to collect and restore.
Phil