I think flax (must be ground so they get nutrition from it and it doesn't pass through whole) is preferable to BOSS because one has omega 3's and one has omega 6's and one is anti-inflammatory and one is inflammatory. I don't remember which omega is the "good" one, but I know flax has the good one and BOSS has the inflammatory one.
I fed BOSS for a while but really didn't notice any difference in the horses. I never fed flax because it was pretty expensive.
Now I feed rice bran and the horses really have glowing, beautiful summer coats.