Well, if you are driving down of have not purchased flights yet, you might want to consider adding a one night stay at one of the three on-site hotels either before or after your Disney stay.
The one night hotel stay will give you TWO DAYS of unlimited express. Now, this is nothing like the Fast Pass system. You simply show your room key at the front of nearly every ride, and you will be riding in a matter of minutes. No having to wait for a return time or anything. If you want to ride Spiderman 10 times in a row, you could do that in maybe an hour or a little longer. Even if the line for spiderman is 90 minutes, you will be on in 5-10, sometimes no waiting at all.
I would go do Universal for two days. If you do it before Disney, check out of your Universal hotel in the morning, pack the car, and go to the parks. Depending on the time of year you go, the parks can sometimes close early (6 or 7pm). After the parks close, or around 7 or 8, I would jump in the car and drive over to Disney to check in.
If you do Universal after Dinsey (which is the way I would do it. It would be a very nice break from waiting in all the lines at Disney), I would check out of Disney bright and early to check into Universal and enjoy your time. With two full days at the parks, you will have enough time to see and do almost everything both parks have to offer while still taking some time to enjoy some nice meals in Citywalk or at the hotel. The onsite hotels are like the Deluxe hotels at Disney, and worth every penny because of how nice the rooms are and the fact that you never have to wait in lines.