That really depends on what you are trying to accomplish. There are so many synergies for Armor and Resistance. If you want to avoid MH charge and Whirlwind skills, then you might not pursue as much AR in order to avoid Battlemage Set. I am not trying to be vague but it really really depends on what you are trying to craft.
Formulating a plan, designing/theory craft, testing, executing, making adjustments, testing etc. It's an iterative process. However, if you have a general idea about what you want your character to do, you can craft around your idea with all the affixes available and 42 slots to fit them. At the end of the day, you may not need much AR or Resistance at all for what you envision.