Well.. Mainly, Beta (on this server) is for testing and will most likely contain unstable versions of code that needs testing, while on Alpha only stable versions will make it..
Alpha is for general public so there won't be any resets on that world (at least not for a year or two..).. But Beta can be reseted at any time (and accounts can be modified/deleted at any time).. If you're not sure which to play, better play Alpha (actually since you don't know what a beta is.. you're bettter of with Alpha world).
And you will be among other players at the same level as yourself. ie-starting out. But you will have the experience of playing here which should help you get a better start.