- What's the difference between the versions contained in the pages?
First one is stable, second one is for development builds, which we call "nightly".
- Why "nightly"?
Calling it "development" or "dev" can be misleading, because "dev" packages have a different meaning in Linux world.
- Are these beta versions, not stable?
We don't do formal beta tests, so no. Nightlies are not as much tested as "stable" versions, but still very stable, because our devs are pretty accurate. Most users are happy with them.
- When I need to download a stable version, which page should I use?
The page from your first link, and download the latest version of Generic PC (no NVIDIA support).