I first purchased AMS on Steam, then yesterday I noticed that the beta version was waiting to be downloaded, so I went ahead and downloaded. the beta.
Do I need both or just 1?
It depends on what you want to do: the beta is updated more often and gives you the opportunity to give feedbacks (good or bad) on Reiza beta forum. The public version is suppose to be more stable (but honestly we never had an unplayable beta) and is the one to use online on public servers.