The first ten days are the whole point of this month
The holiday overlays at both resorts are fully installed by the start of December and do not change between then and the new year. The crowd and the price change enormously. A trip in the first ten days buys identical seasonal content to a trip on the twenty-eighth at a fraction of the cost and a fraction of the queue.
That is not a marginal saving. In Orlando the difference across that boundary covers hotel rate, ticket tier and Express Pass simultaneously, and for a family of four across three park days it is the largest date-driven swing anywhere on this calendar.
The only argument for the second half is that you want to be at a theme park at Christmas specifically, which is a real reason and should be recognised as an expensive one rather than dressed up as good planning.
Universal does not sell a hard-ticket holiday party, and that changes the budget
Neither Universal Orlando nor Universal Studios Hollywood runs a separately ticketed Christmas party in the way visitors coming from the other Orlando resort expect. The holiday parades, the tree lightings, the Grinchmas show and the Hogsmeade projection show all run on regular park admission.
The practical consequence is that the holiday season costs less at Universal than the equivalent elsewhere, and it also means there is no cheap-night, expensive-night structure to arbitrage. The lever is the date of your park ticket, not the date of an event ticket.
The exception is New Year's Eve, where Universal Orlando has historically sold separately ticketed CityWalk celebrations with their own pricing. Those are the one holiday product here that needs booking rather than turning up for.
Capacity closures are a real risk in the Christmas fortnight
On the busiest days between Christmas and the new year, Universal Orlando parks have historically stopped admitting guests, including guests holding valid tickets who arrived late. It is not a common event across the year and it is a recurring one in this specific fortnight.
The defence is arrival time. Being through the gate before opening on those days is not a tactic for shorter queues; it is what guarantees you get in at all. Anyone planning a leisurely start on the twenty-eighth is planning to be turned away.
On-site hotel guests get early park admission, which on those days is worth more than at any other point in the year. This is the fortnight where the Express Pass hotel inclusion and the early entry together most clearly justify the room premium.
Southern California's school break lands later than Florida's, and Hollywood follows it
Southern California districts generally break up in the last full week before Christmas, which is later than many of the states that supply Orlando's holiday visitors. Universal Studios Hollywood therefore holds a lower crowd level further into December than Universal Orlando does, and its peak is shorter and steeper.
That makes the second and third weeks of December a better window in California than in Florida. It also makes the days right after Christmas worse in California relative to capacity, because a compact park takes a concentrated local crowd badly.
Grinchmas at Universal Studios Hollywood is the seasonal draw and it is included with admission. Its show schedule is published for each cycle and the shows fill, so plan the day around the performance times rather than fitting them in.