Your links have two completely different versions of KiCad: the testing version of the stable branch 5.1 where there are only bugfixes; and the “nightly builds” for 5.99, the unstable feature development branch.
If by “next release” you mean the new features you should use the official nightly builds which is nowadays compiled with MS Visual Studio, hence “msvc” in the file name.
You can use either the full or the “lite” installer. The full 1GB installer has the libraries inside it. The lite has a run time (installation time) option to download the libraries separately. Both lead to the same setup if you choose to install the libraries. As Paul said, 5.99 is automatically installed separately from the stable 5.1 so you don’t have to worry.
You should keep up to date with the nightly builds especially if you find bugs and report them. At the moment bugfixes and other small changes come in every day. Updating the libraries every time isn’t necessary, therefore I recommend the lite installer for updates.
The most important thing to remember is to not open and save the original v5 projects with 5.99 because you can’t go back.
At the moment it’s pretty safe to edit a project with a new nightly build. Even if there’s a new bug which prevents working, you can install a bit older version which didn’t have that bug. There will be crashes and there’s no guarantee that your files will stay OK, but chances of losing all the work are small. Save often and use a version control system if you do actual work with a project.
You can follow the development status in Post-v5 new features and development news. (The link goes directly to a post which may be the first relevant for you, otherwise the thread is too long to read.)