Our 1-day trip to UH almost soured us on Universal. As we were meeting relatives who live in LA and have been to UH many times, we ended up arriving late on a busy day and did not get EPs. Needless to say, we didn't get to go on many rides. When we were looking at an Orlando trip, we were not going to pick UO because of the UH experience. But because the kids had become such HP fans, we decided to give it a shot and fell in love on our 4 night trip. We've now been 5 times in the last 4 years, and haven't been back to DW in that time.
So if you do go for a day, arrive 30 minutes before rope drop (even if you have EPs). You can do a lot more in those first couple of hours, and plan for a long day to squeeze in as much as possible.
Given how much time you've spent at Disney, I'd suggest doing both days at Universal. As mentioned above, if you stay at a deluxe resort for one night, you get EPs (think unlimited Fastpasses at Disney) for both your arrival and departure days. And the cost of the second day at UO is much less than a day at DW. If you can get an APH rate at the resort, you might even save money upgrading on ticket to an Annual Pass.
but universal is completly new
