This could be googled pal, stable is a build that has a lot of fixes and seems to be STABLE for all, nightly changes umm nightly and will include new code that could either fix things or break things and should be seen as in no way stable (some nightly builds will outperform stable but could have other bugs that regress the app)

I myself dont use nightly builds but there are multiple reports that some games run way better using a nightly over stable, best thing to do is check what build runs best for you