This is a very tough comparison because these are 2 excellent themes for WordPress.
Both themes are equal in terms of speed. They are some of the fastest themes available for WordPress.
If comparing the free versions, Astra crushed GeneratePress, which is more of a demo.
But when comparing paid versions, GeneratePress is less expensive, has 1st class support, and has some unique features that make it a bit easier to use for a power user. GeneratePress has more flexible header options including a vertical menu, something that Astra lacks.
Both themes have demos, but they approach them differently. GeneratePress's demos are more starter styles vs fully built out websites. Where Astra's are all fully built out websites. Which is better depends on you.
It's impossible to say one is better than the other, and it wouldn't be fair to choose one because everyone has different needs.