If you are talking about the Monstrack mirrors, definitely get the long. I wasn't sure myself, and they told me about 90% of orders are for the long. They aren't too long, just right. They should be called "regular" and "short" in my opinion. The short ones wouldn't give you very good rear visibility, I don't think.
i have the monstrack "long" mirrors in black and love them. i wouldn't want them any shorter. any shorter, and all you would see would be your arms. the long terminate at the same point as the bar ends. the machining, quality and fitment are A+++. go to products review for pics. these were one of the best mods i did. 8)