Not knocking this product, but I personally prefer a trackball on top with buttons to either side. My thumb get very stressed out using the "on the side" arrangement where the top I can vary how I interact with it more (hover hand, rest a finger on it, flick it with index/middle, etc).
~~Either~~ Both are preferable to the standard mouse for me, though!
~~Either~~ Both are preferable to the standard mouse for me, though!