I would for sure do the two stays. We have done it both ways and staying onsite at Universal is incredible. Nothing ever beats Disney in my book because I like the parks better, but the hotels at Universal are so much nicer than any of the Disney ones. The front of the line access that you can use over and over is just amazing. It is like having fastpasses all day for so many of the rides (Not all of them though) and you also get early access to Harry Potter.