I've never seen a Penn-plax cascade 1500 in use. However it is a bit different than a Magnum 350.
-A magnum 350 is more or less just for heavy carbon filtration or Micron water polishing, both giving you crystal clear water. It is not meant to be your primary biological filter.
-The Penn-plax is meant to be an "all in one" type filter. Similar to a Fluval, but doesn't appear to have as many options for filtering material as a Fluval.