I haven't gone the week between Christmas and New Year's, but I have gone a few days before Christmas. Once was without Express and once was with. The thing to remember about Universal, is that even with the Express queue, their lines still move, and a lot of their attractions are high capacity. Which makes standing in a longer line a less painful experience than it is in WDW these days. And if this is your first visit to the WWoHP you really aren't going to mind a 30-45 minute wait at Forbidden Journey and Gringotts. Especially, FJ when each of the rooms has the character's dialog running, and you're going to want to listen to all of it, and look at all the details.
So shockingly, my recommendations are, don't worry about Express:
1. Make sure you are at the gate, ready to go before the park opens at 8AM. US side, they let you into the front area, and then usually remove the barrier about 7:50 and we beeline to Minions. From our experience, the first 2 hours are manageable. We tend to hit non-HP rides first, and then go into a Harry Potter area after the first "wave" has hit. Usually we do, Minions, Transformers, Mummy and now Fallon before heading to HP. IOA side we do Spiderman and Kong. Sometimes we start with Hulk, but we've reached the age where we don't always have to do the coasters. Obviously, you can't start out at both parks but Kong and Minions are the two rides where you can get bogged down in an interminable wait. So pick which ride sounds most interesting to you.
2. For non-HP attractions: Spiderman, Mummy and Men In Black have single rider lines. FJ and Gringott's do too. Good for re-rides, but still do the queues once.
3. If you can stay late, late afternoon and evening hours the lines drop off. Use the busy part of the day to watch the HP entertainment, play with the wands, tour the shops, etc. Keep an eye on the app, to look for lulls in wait times.
4. Fallon and Fast & Furious have a virtual queue and you can reserve a time via the Universal App on your smart phone.
5. Butterbeer is available in the HP restaurants, you don't have to stand in the long lines at the wagons. We usually go in the Hogs Head in Hogsmeade and the Fountain of Fair Fortune in Diagon Alley.
The key is to know what you are getting into. Have a plan of what you want to accomplish. Don't show up at noon, and then get in the first line you see and then spend a half an hour trying to figure out what the next thing you want to do is, lazily shop, and eat. The "old-fashioned" way of touring still works if the park is mindful of its operations, and I have found that Universal is. Watch the TM's at the load and unload, they are moving to get cars dispatched.
