I really want to play The Binding of Isaac but I'm not sure which version to get, so what I'm asking is this:
Should I just get Rebirth? Is there any reason for me to want to play the original one first?
The original Binding of Isaac has nothing exclusive, so I think you can start with Rebirth, which is a remake of the original one. The plot and the gameplay are pretty much the same.
The newest version has more characters, more items, more achievements and more challenges, and quite everything that was in the older game is present in Rebirth. And soon will be released a DLC with other contents.
Moreover, the new game has better performance and less bugs, so I recommend you to play Rebirth.
Of course the original Binding of Isaac is a great game, but has some limitations due to Flash, so the game wasn't released with all the features that McMillen wanted.