I can't stand how slow heavy armor makes you. Sure you can take a few more body shots (not head shots, those aren't affected by armor) but you also lose the ability to run and dodge effectively, meaning you take way more than it's worth - in my opinion.
I've actually found that heavy armor makes it harder, at least for me - too difficult to escape sticky situations. I've been running light or medium armor with the scout passive and playing like it's a stealth game - sneak up to an outpost, lob an eagle/orbital/impact grenade/etc in to clear out the fabs and bots, then quickly clear any objectives and get out before a patrol arrives