We would love to get the express pass by staying onsite, but it is a lot more expensive than staying off site. Is the express pass truly necessary around Jan 5/6?
No problem. The main difference with the Express pass that you purchase in the park and the one that comes with your on-site hotel room key is that the purchased Express pass is good for only once per ride, whereas the Express that is included with an onsite stay is unlimited.
If you purchase a HP package that includes tix and a stay at a partner hotel (I think), you can qualify for early entry (but not Express privileges). Early entry to WWoHP can be very advantageous if Harry Potter is your thing. More info in this thread: http://www.disboards.com/showthread.php?t=2687755&highlight=partner+hotels&page=2