I would go with 2-3 nights (depending on your arrival time) at Universal and the rest at Disney. We got a 3 day park-to-park pass for Universal and it was the perfect amount of time there for us. We hit each park one day (and a little back and forth with the HP attractions) and then used the 3rd day to go back and do our favorites again. We would've been fine without the 3rd day, but the difference in the ticket price was minimal, so we went for it. We stayed at Cabana Bay. It was a fun resort. There is a bus that takes you to the parks or you can walk. If I were to do it again, and my budget allowed, I would go for one of the higher-end on-site resorts.
I would allow at least 4 nights at Disney if you really want to experience "the bubble" effect. For me, I find anything shorter just isn't enough time for me to let go of the outside world.
It sounds like a fabulous trip!! Have a great time!!