This is a very tough comparison because these are 2 good themes for WordPress.
They are also 2 of the older, more established themes that you could choose from.
Both themes are equal in terms of speed for real-world usage. They are some of the fastest themes available for WordPress.
Both themes have demos, but they approach them differently. GeneratePress's demos tend to have a very clear design style. Where Astra's are all fully built out websites that offer more variety. Design is subjective, which is better depends on you.
It would be fair to say that GeneratePress is more developer focused and Astra is more mass-market. Thats not to say that a developer wouldn't choose Astra, its just the primary user type that they are made for.
It's impossible to say one is better than the other because they are made for different users, and it wouldn't be fair to choose one because everyone has different needs.