IIRC, the Mustang also sold a million copies in the '65-'66 period. Right now, GM is on track to hit that number of Volts (and Cadillac clones) in around seven hundred years. There will still be classic Mustangs running long after the last Volt bursts into flames.
Based on longevity alone, the Mustang wins the environmental impact contest.
In addition, the 64 1/2 to 66 Mustangs have the 'it' factor that a modern Cadillac can't touch. Park a nicely maintained or restored original Mustang in a parking lot and almost everyone will give it a long once-over (and most guys will circle the car in admiration) while a new Cadillac won't elicit a second glance.