I have been on both but it has been 2-3 years since I have been on the Northwestern Fjords tour. My preference is for the tour that gives me the biggest discount. This year it was a 2-for-1 offer through the Iditarod that gave me 2-for-1 on the 6 hour Kenai Fjords tour.
Having said that, yes you do get a chance to see more wildlife since the tour is 3 hours longer. As to seeing more glaciers, not really. With the Northwestern tour you see the Northwestern Glacier, with the 6 hour tour you see the Aialak or Holgate Glacier.
You can't go wrong with either one. They are both great.
If glaciers are your main interest, then the Prince William Sound tours from Whittier emphasize glaciers (whereas the Kenai Fjords tours emphasize marine wildlife).