Paracord

 - Better strength*/diameter ratio.  550lb paracord is 5/32" (4mm) in diameter generally.  Twisted poly of a comparable strength is around 3/16" (4.75mm).  So the paracord will pack smaller.

 - More versatile.  You can easily divide it into multiple ropes.  Its form also makes it much easier to braid than twisted poly if you need to make a stronger rope.

 - Easier on your hands -- If you've ever tried to hoist a heavy pack with twisted poly you know that stuff eats skin.  The smoother paracord is less hand-shredding.

Twisted Poly

 - Cheap-er (about half the cost) - [Paracord 1000ft/$50][1] (0.05/ft),
   [Twisted poly 1200ft/$32][2] (0.026/ft).  I say cheap-er, because
   paracord isn't exactly what I would call expensive.

*Strength = maximum load strength.  
(I know people are looking for weight comparison too, I'm going to post those when I break down and spend the $ on twisted poly)


  [1]: http://www.amazon.com/Black-Parachute-Paracord-Military-Specification/dp/B002HJ1CK4/ref=sr_1_7?s=sporting-goods&ie=UTF8&qid=1338399350&sr=1-7
  [2]: http://www.uline.com/BL_3912/Polypropylene-Rope?pricode=WO439&gclid=CInEw_DFqLACFQSxnQodLUXcWA