Honestly: Buy direct because you want to buy Poly points and stay at the Polynesian, probably in studios, 80% of the time. Buy direct if you're a Hawaii family and want to stay at Aulani every other year 80% of the time.
Do not buy anything else direct in order to get the contracted exchange rate for Disney Collection, because the contracted rate is going to be higher than the cash rate with discounts, and is a poor exchange value based on the value of your direct points.