Each one in the grand company store is a different set of 3 pieces(helm, saddle, boots), the highest ranking set has more armor, decals, and a crest on the helmet. Each grand company barding is unique(Maelstrom is all gunmetal with red flags and decals, Flames is steel with gold decals and black flags, Adders is lighter colored steel with yellow feathers and gold flags). Once you finish the story you can switch grand companies, but it takes 30 days and some gil to switch again afterwards.
There's also a set from using the mantles from killing Odin and supposedly there's a set from each combat class coming out at some point too.