Both places make it totally worth your while to stay on property. Disney has Extra Magic Hours where each day, a park will open either 1-2 hours early for hotel guests only or stay open up to 3 hours late (Leaving Magic Kingdom @ 1AM rocks!). As it should be, Fast Pass at Disney is free for everybody on a first come first serve basis. You are also able to use the entire Disney Transport shuttle service and basically never need to drive the entire time. There are other various perks as well but we'll save those for another time.
Universal allows all hotel guests access to thier "express pass" system. This is great for hotel guests but LAME for day trippers as it isn't available to them (it used to be available as a upcharge for day guests).
Universal is so lame at this point you don't even need it anyway. You can do both parks EASILY in 2 days without the express pass because turnstile clicks are so low. The only exception to this would be Halloween Horror Nights which ONLY allows express pass as a upcharge.
We can talk more later as you get closer to planning a trip. My wife swears I could go into business doing this for people lol.