Well the design is much different, basic pencil/graphing pencil vs writing pencil. The ergonomics would be the main thing.
The Sharpwriter by Paper Mate isn't all that bad but it isn't designed for long term use. They bow or break in pockets or from frequent use and they use pretty crappy lead that doesn't like hard writing. Also, the eraser is really low quality and won't erase cleanly and leaves a lot of dust.
If you like the design of the Sharpwriter I would suggest a Zebra M-301 or Pentel 205 as a good step up in quality for pretty cheaply.