you can install it to either or both, depending on which program you use the most or want to use these products in.
You can also install it to a RUNTIME that isn;t even in Poser...one that resides on your hard drive...which can be named whatever you want so you recognize it...many choices.

The easiest way is to just run the DAZ installer...then choose which Poser version you want the files installed into. All will go to the correct locations automatically, and then enjoy. Be sure to read the readme page that is generated at the end of the installation. It tells where to find the products within Poser and other tidbits.

HTH ... later , we can discuss how to set up separate runtimes, and so forth. Right now it sounds as if the more straightforward path will be the best until you are more comfortable with installing these files.

Humankind has not woven the web of life. We are but one thread within it.
Whatever we do to the web, we do to ourselves. All things are bound together.
All things connect......Chief Seattle, 1854