I'm not sure of the price difference, if you call Aulani direct they will give you all of the pricing info (they were very helpful and great).
We booked ours very soon after booking the resort, probably 6 months out, but that gave us any date while we were there to pick from, again they can help when you call.
We received 2 CDs with photos on them, I'm not sure what a "usable" photo would be considered...to me that is subjective (I'm looking at 8 we had printed and framed, with plans for several more, just need to find space). We got more than enough to print, frame and remember our trip. We got family shots and individual kids shots...compared to a local photographer that we had a previous session with, I think we got as good of a deal if not better (location, location, location).
We didn't do a clothing change, not sure how that would work, but assume it would take time from the session.
I was very unsure if it was the right thing for us, it's not inexpensive, but I am so happy that we did it. We are hopefully returning next summer and I will book again without a second thought. For us it was worth it.
It is just a cd with the photos on it with a release to have them printed where you chose, so keep in mind that is an added expense.
Let me know if you have any other questions!