Totally different head sculpt, no shoulder armor, different straps as you mentioned, and vastly different paint on the outfit (this all from examining the TTT Legolas which I didn't even know existed until you mentioned this).
The ROTK Legolas has some different, thinner straps on his torso. And he has a silver shirt instead of a black one. Other than that, he's pretty much identical to the Rohan Armor/Helms Deep version (he has the same head as the Helms Deep one too).
I think He looks pretty cool, but no better than the other versions. I would only buy him if you're into variations.