Its a fairly small difference between alpha and beta keys. Alpha and beta keys gets access to the beta plus the full version. If there is later additional content or features that are not included in the base full game alpha keys get access to that for free beta would not. At least that is my understanding of it.
I would question your definition of fairly small difference, but, If you are saying that right now, the game is the same for both people who bought the game, alpha or beta you would be correct... there is no immediate benefit or difference to having either account.
The difference is in the license..
The Minecraft alpha license includes All Future Versions of the game free of charge, Minecraft 1.0,2.0,10.0 eleventy hundred point zero. All future versions.
Beta licenses have removed these terms, however you still get a small discount... and are still priced more than my offer, and you can't purchase with bitcoins.
To me that is a fairly BIG differentiation.