Effective health is significantly more important if you are doing hard modes. If you are doing normal difficult only I would say using sacrificing some effective health for avoidance is ok. Rather than going for hybrid gems with dodge or parry, I would say use offensive gems hit or expertise. Which would help compensate for some threat loss in your spec change. Expertise is better than hit. If you do get avoidance, dodge is better than parry.
If you are doing hard modes though I'd avoid going for anything other than effective health until you have it on a more comfortable farm status.