First of all, Defense is always better if you haven’t reached the Block Cap, the old Uncrushable of 102.4%. Defense is the most efficient way to reach the Block Cap.
Once you have hit the Block Cap, to get the best avoidance, and taking into account diminishing returns, you should keep your dodge rating roughly equal to your defense rating.
We can also conclude from this that after Block Cap, Defense gemming is generally going to be a poor choice.
This is something you considered AFTER you are uncrittable. Remember your first goal must be the Crit Cap.