I feel as if titans should have significantly more armor. The extra defense from the Titan mastery was not very noticeable in my opinion. I don't know how it is late game but, as far as the beta showed, it was very underwhelming.
The armor cap for all classes in the beta is 160, but if you look at your characters stats you will see that titans have the most defense(which is calculated on top of your armor rating), while hunters have agility, and warlocks have recovery. Every class has their own perks but titans are definitely the most difficult to kill if they have their maxed out armor rating. :)