Always thought the first starting point is for those who want an alternative story as Norway winning or keeping an anglo-saxon England, don´t know why you bother so much. Second point is just a few months later, it´s not like you´re shortening your game terribly.
Exactly this, the first bookmark is not worth your time with William, since even if you conquer England by year end and take all counties, you'll end up with a 1000 piety but no levies or taxes from your vassals or self for 20 years. The conquering itself is a cakewalk, you outnumber Harold 2:1 without mercs, leaving you the boring process of sieging and playing tiny levy fly squat.
Harald can be ignored for the most part, his age will kill him if a quick scrap doesn't.
Also with the second bookmark you start with medium crown authority and Seniority succession, which is quite the headstart.