I have been to both places in the past year, and am two months away from doing almost exactly what you're suggesting. I added an extra day to do 5 days at Disney and 3 days at Universal. Last year when at Universal with my wife (no kids), we found 3 days to be too much time. We covered everything we wanted to cover in 2 days, and the third day was mostly spent watching the lesser shows and re-riding some things. But, now we're taking our kids this year, and figure that we'll need the extra time to do the kiddie stuff. We stayed on Universal property, and are doing so again, for three reasons...first, we are staying at Royal Pacific, and you can walk to Universal in less than 10 minutes (or take the boat if you don't mind waiting at the dock if it's not there)...second, the Express pass..third, the hour early entry to Harry Potter beats the crowds! Those reasons alone make it worthwhile, although then you do pay a $15 per night parking fee.
For Disney, we are also staying on property again, but mostly because the kids have always wanted to stay at AK Lodge, and we were interested too. We are driving down from the MD area, so the bus service onsite isn't really that important to us as we'll have a car (and the
AAA parking pass!). But, the free parking for being onsite, the extra magic hours, as well as room charging and delivery, are all nice little perks.
For us, Seaworld wouldn't be much of a draw. Fish are fish...a whole day of fish vs an extra day of magic at Disney? I say Disney. Personally, I think DHS can be done in a day, as can AK. Epcot is definitely at least a day and a half to really enjoy it without being rushed. MK can be anywhere from a day to two days. With park-hopping, it really gives you flexibility to do what you want when you want.
I wouldn't recommend trying to stuff all of Disney into 3 days. Universal can be done in 2 days without kids, and with the Express pass.