That even tho there is a pro and contra in both of them, you end up with EH cause of the way the endgame instance is designed.
Over-emphasizing avoidance over Effective Health leads to fights in which the tank cannot survive bursts that inevitably occur. With less avoidance and more EH, these fights would be better survivable because the tank would live through unpreventable bursts. But at the same time, over-emphasizing Effective Health over avoidance leads to fights in which the tank is frequently faced with near-disastrous situations which cannot always be overcome. With less EH and more avoidance, these fights would be better survivable because the frequency of those dangerous situations would be reduced.