The "Early Adopters Builds" as we've called them are basically a continually updated Demo of the game intended to allow people to play the game and give us feedback without having to commit any money to the project. We simply want the make the game the best it can be, which requires getting as much feedback as possible.
All of the pre-purchase options will get you immediate access to the latest full version of the game(updated at the same time as the Early Adopters Builds are), along with a whatever incentives are listed with the particular option you choose. This is a true pre-purchase, so for just $5 you are purchasing a final version of the game, not just access to current full version of the game. Not to mention that I've learned a lot from Tripwire's updating policies, so you will also continue to receive additional free content after the game ships.