Funny you ask this now since my DH, teen daughter and a friend are flying back from there as I type.
As a family we were just down there for Christmas and stayed on Disney property, Old Key West. They are like little condos with full kitchens so we were able to save some money by occasionally eating in our room. But since we were there with all the extended family, my husband and daughter did not make it over to Universal to see the Harry Potter thing.
So for Spring Break he promised her and a friend to take them since they are all huge Harry Potter friends. They actually stayed 2 days at Universal and since my daughter's friend hadn't been to Disney they went over there for a couple of days.
I really don't know the cost breakdown of the air, hotel, park tickets etc since I didn't go and didn't pay attention. I can tell you that for 5 days it was over $5000. Now, keep in mind they had 2 rooms, one for my husband and one for the girls (2-18 year olds) so that added a little bit of the cost.
They stayed at the Portofino? at Universal. I found it very interesting that being spring break and high volume weekend, the hotel restaurants closed early. My husband said that they had a hard time finding some place to eat at 7:30 pm on a Friday night! In his opinion, all Universal really had to offer was the Harry Potter attraction. I haven't talked to the girls about that anyway.
They went over to Disney on Saturday and stayed at Coronado Springs, which my husband said he wouldn't stay there again. Now keep in mind, every time we go to Disney we always stay at Old Key West, so I'm sure having the room of a small condo to a 'regular hotel room' was a change.
We also always get 'Park Hopper Passes' which allow us to go from park to park even in one day. In my humble opinion, you could do the Disney Hollywood park (used to be the old MGM park) in a 1/2 day. But that's my thought. I could then spend a day PLUS that 1/2 at Epcot. And then a day at Animal Kingdom and a day at Magic Kingdom.
We have never stayed off property when we go down. I'm sure you can find cheaper hotel rates off property, but staying on has benefits. Busses pick you up at the airport and take you straight to the hotel. There is always transportation between the on property hotels to all the parks. By staying on property, you get special access times to the parks. For example, last night Magic Kingdom was open until 2 am for those staying on property. And other days, you get in early which is a HUGE benefit for some of the more popular rides and attractions! Not to mention little things like going back to rest before dinner or if you buy something, you don't have to carry it around with you, you can have it sent back to your room while you continue to enjoy the day.