Power Draw is different in regards where there is a limit to the benefits. Your proficiency increases based on the amount of damage you do but also relative to your weapon master.
Each rank of weapon master allows the soft cap to go 40 more proficiency points which can be allocated from the character screen. After reaching the soft cap, you can still earn points but it takes far more and can only be earned through the "natural" method by doing the damage.
If you keep your weapon master at 1, you will still be able to earn points past the 100 cap well into 300, but you won't be able to allocate points via the character screen. Additionally, it will take a longer time to earn a point naturally than if you had a weapon master of 5.
I say power draw is different because the damage bonus you receive is based off the bow you use. You will do the same amount of damage with a PD 2 requirement bow if your character has PD 7 and PD 10. Max is 140%.
When leveling your character and polearms, you are best off always couching your lance since yields the greatest damage bonus. Using a faster horse such a courser will give you the most opportunity for weapon proficiency increases but at a greater risk due to the lack of barding.