For about $5 you can get a subscription to touringplan.com that has recommended Universal/IOA touring plans. We had also been to the parks before, so started with these and did some customization, taking off things we weren't interested in, plugged in our date (it was just a one day park to park for us) and looked at wait times.
This convinced us (and if worked out great), that for the week before Christmas week we didn't need either early entry or express pass. Even at this less busy time though there were just a lot of people in the Diagaon Alley area. Waits weren't bad (practically walk on for most things), but for looking around in HP areas I could even then see how early entry would be nice to look around without so many bodies there. And that was a crowd level 3 day.
We did the same thing for a later time in March (peak spring break season) just to see what things would look like then. Your dates might not be as crowded as ours being earlier in the month. Based on that (if we decide to do a day at Universal again), I decided it would be worth it to use to stay in one of the deluxe onsite for the night before (first day of trip) before moving to our other place so we could get both the early entry and the express pass. And I upped the budget for this (lol -- very cool place, but it sure is expensive to do just one day here // better value of course for more days, but one would be all we are interested in). DH hasn't seen HP areas before, and if he comes on the trip we may want to add this as DS would love for him to see these. A big thing here though is that DS would do some stuff we wouldn't and with this our waits for him at exits will be more like four minutes / six minutes here and there vs. 20 minutes, 25 minutes, etc. making for a more fun day for us. DH in particular really hates lines and waiting around and typically skips theme parks entirely. If it was DH only actually, I don't think there is anything except the HP areas and maybe Jurassic Park area that he'd actually have any interest in.
You might want to see what the waits look like for you for what you would do, and if that looks reasonable, or if you'd want to spend extra dollars for express passes or onsite (it is a pretty big premium -- deluxe was $180 more for our one night). It was interesting that staying at the Universal hotel that doesn't offer FOT and express pass cost (We probably won't do anything more than one time) was very similar in cost to staying a the deluxe hotel (deluxe hotel for the three of us maybe $40 more expensive). That was so close in cost though that for that I'd do deluxe, so we could hop on things if we wanted too more than once with the FOL pass. Both hotels look nice and would be very adequate for us, but I'm guessing maybe the deluxe hotel is maybe a little nicer too. I'm really, though, more of the three star vs. four star hotel kind of person - don't like higher food costs and extra add on costs most four star hotels have.
