The avoidance and threat stats are similar so I'm more looking at this from a pure EHP standpoint. However if the 4pc bonus is worth more than the extra armor and stamina, I would obviously go with either the tier chest or helm.
The T9 4-piece set bonus is not all that great. The T9 pants aren't all the great either. In making the largest upgrade though, I would probably suggest going with the pants. You do not want to replace the helm because of the expertise (you're sitting at 20) and the chest pieces are very similar with the one you have giving you more EH which is really what you should be shooting for.