These are, one might say, different games. Both have a different engine. In terms of functionality, the preference of course goes to S&Box. Almost everything can be implemented on it. What can I say about Garry's Mod - Lua language does not allow you to create various modifications so widely, and it is less optimized than C#. Nevertheless, I personally will stay in Gmod, since it has its own unique atmosphere and servers, and everything will be different in S&Box. Everyone's opinion is here, but most likely there will be a lot more interesting modes in S&Box than in gmod, but this is unlikely to change my decision. Of course, you can try it on the release, but only just.