health/armour balance

I just started playing POE and wondered about the balance between health and armour.
Most builds I've seen prefer health over armor, why?

This is a build I made, and noticed I had a lot of armour.
Should I switch some of it for health?
And are there any other comments for the build?

http://www.pathofexile.com/passive-skill-tree/AAAAAgEBxthYY1BQjM_2SF8_VEnnUu8Oud3UjxkuJ-2sqnKpfNlR5rQvQT_ZYVcNkc57jCFVOQ6pcyECKvbSIZ7NpzB25_e-FCBQRxiRyZimZA_MDjw6UoZgVgRhUmCI7hQyCSi1706i6hLhYSGHapBsqZSE2Ud-ZU0UUqvFLPtfOd3jypCZK0CgIWB08RjbQ5wOrSmlW6_dDTqzNYa5zZ2q_MVnoId2Sn0FLWBLeu907cRYJ9VW-ooiwPN89cCm7g4j9u_wWkjfv7_VRJ6wc-y28fRTu76n1e3STcAPoJ2cJ5_L