Cost is a personal decision, but we stayed off site and decided to have dinner at CityWalk. To say it was crowded was an understatement. It is bonkers. We parked on the top floor of the parking garage, one of several. I would stay onsite even one or two nights, just to get the express passes and closeness of the parks. It is several walkways and elevator to get to CityWalk, then to the parks from the parking garage. With crowds, it's going to a slower walk.
Do the math, then keep checking cheaptickets/orbitz or other travel agents for a discount, as I doubt any of the onsite ones will be giving any discounts.
While you'll pay for parking regardless of garage or hotel parking, staying onsite is still a bonus, that you get early entry, which is a great way to start a park, and unlimited express passes for all in your room, for day checking in through day checking out. That's why some do a one night, to get 2 days of the express pass. Has nothing to do with your park tickets. Try
undercovertourist or maple leaf for discounted tickets. Both tend to be cheaper then Universal, and both include tax in their price, while Universal adds it when total. So check and see which is better for you.
If it's only 2 of you, then I'd say brave it, and don't stay onsite. If you have 4, then based on Damo's 150 per day for one person, the hotel rate would be almost the same.
We enjoy USO, but like anything around the Orlando parks during the week after Christmas is just busy, busy, busy.