Armor in XCOM is described as being either light, medium, or heavy. What are the effects of wearing a particular type of armor?
The only gameplay difference is the Assault Class's major rank ability: Extra Conditioning.
For all other intents in purposes, armor type doesn't matter. Stick your Snipers in Titan, or your Heavies in Archangel - they'll get the maximal beneifts either way.