The BETA is "newer", but it's "beta", so not an official release.
For NASSP (as for every other add-on) it depends whether the add-on was build/released for BETA or for 2016 (or any other older) version of Orbiter.
Usually the developer(s) notes for which version of Orbiter the add-on was compiled for.
The "newer" is Orbiter 2016 beta. If you want to use NASSP 8.0 it has been specifically compiled for Orbiter 2016 beta.
Now the rule for most other addons is that they should be compiled against Orbiter 2016 P1. It is written somewhere in the beta SDK doc. Compatibility between both versions isn't guaranteed, because some API functions have been modified in the beta.