I think you should give it a shot. Personally, I have never been to Universal Orlando but I added US Hollywood to a Southern California trip and was really pleasantly surprised. It's a very different experience from Disney, but if you are the kind of person that enjoys rides, shows, and the whole theme park experience, it can be really enjoyable. (We actually only planned one day at US Hollywood, but ended up adding a second day because we had so much fun!)
I think that 7 days is too long to spend at Universal, but I also think that spending only a couple days at Disney would be torture.
I don't think I could ever go from Universal to Disney and/or back again. It would have to be separate trips for me. Perhaps you could plan other Orlando-area activities in addition to US if you plan to stay for a week?