Let me preface this by saying Marino is my favorite athlete of all time.
But I think Manning's sustained excellence is enough to supplant Marino as the GOAT.
But don't get it twisted, both in their primes and they are playing under the same rules? I'm taking Marino every time. A prime Marino is still the best QB of all time to me. That achilles injury derailed his career and he never had the team success that other greats had because of his wack ass supporting cast.
Can you imagine what Marino would do in this modern day NFL where you can't even touch receivers without getting called for a penalty? No lie, he might put up 60 TD's and 6,000 yards.