Since game is in Alpha stage, the only difference is that you either supporting a development or not. Further development requires additional paid professional staff members, thus the price rises accordingly to the content already developed.
The last patch of Alpha version will become a free Demo-version. You will be able to download it at any time and play single-player or multi-player, but only with your friends and servers using a Demo-server. First Beta version, Release version and all further updates will most likely be available for the paid members only.
As soon as you feel the game worth the money, you should buy it. Unlike Minecraft, this game will not be intentionally spilled for pirating, as it is already requires registry authentication to play on most trustworthy servers.
