I havent seen Poseidons Fury yet /gasp
I will be staying onsite Dec I will have to see that.
The site to see the hotels is universalorlando.com incase you did not know.
They also have them listed on hotels.com and expedia etc I believe you could check there for a good deal. When you stay onsite you get 2 days of express, the day you check in and check out.
Also would depend if your going on a weekend depending on the crowds. I went during offtime and still there is a bit of a wait and between 2 parks there is a lot to see. From what I have heard Mondays and Tuesdays are less busy but I am not too sure when kids go back to school this year.
Whether its worth it or not is pretty much up to you. But if your kids are begging to go and you can give that to them imo it would be worth it!
If you have any other questions can post here or Universal forums and I am sure there will be plenty of help if you need it.