The difference between the two types:(1) Price. About $30 or $40. (2) The 580's have a better glass clarity to them that gives you a better under surface in the water. Also a better scratch resistance to them than the 400's.
I have been looking at upgrading my Fathoms to 580's, but with 3 pair it is hard to justify a good pair even if there the 39's basic model. One of the pair of Fathoms I did replace my lenses after three years of use. Not bad. I have two pair of blue mirror/gray lense for everyday use (also for the deep sea trips in the big blue) and a pair of green mirror/amber lenses for when I hit the emeral green waters of Florida (or Corpus). Also have an old pair that are amber lenses for cloudy days.
Check out www.campmor.com
. They have 580's for $120. Thats a good deal.
I hope this helps a bit.