The Evolutions is more to the West End Games look for the character, and its original designs I think...http://starwars.wikia.com/wiki/Storm_commando
So for me, that's their ultimate "official" look, but I think the others work as maybe variant armor. I see a Commando unit as having some flexibility in their gear, even in the Empire, and so they're a little more likely to have some diversity. Some may prefer Scout armor, others may like something a little tougher, and so on. The Evolutions figure though, to me, seems more representative of the characters. No surprise it's also the most distinctly different sculpt though too.
It could be argued different units within the Storm Commandoes have different armor too, for a variety of reasons...
I treat the original "Kashyyyk" Trooper that was a repaint of the VTSC Scout as a Spec. Forces trooper... I treat them as pathfinders among the Scouts. Sort of a recon/special unit separate of the regular Scouts. Armor's the same, just different colors. It helps me BS the many I bought into my displays too.