YOU WILL BE SO SPOILED!!! We just returned from the Hard Rock Hotel (having stayed onsite at Universal 5 times), and there is no better experience! Touring both parks is a piece of cake! No getting up and running out of your hotel so that you can be there when the park opens, no extreme planning so that you can see and do everything--it is an absolute pleasure!
One piece of advise however--when you enter IOA, the Hulk and Spiderman (probably the parks biggest attractions), are to your right. 95% of the people entering the park go the right--we always go to the left and work our way opposite of everyone else.
As previous posters have said, you show your room key and proceed to a different line. Your wait for most rides will be minimal (5-10 minutes). We've done many rides over and over again simply by getting off the ride and going back to the Express line. There is no limit in the amount of times you can do this!
When we've done Disney and Universal in the same trip, we always save Universal for last. After "extreme planning" when touring the Disney parks, touring Universal is a pleasure.