Oh wow, Universal is amazing! Personally I took two days to get through it. One day per park. This was in September 2009. My brother in law went to just Universal and did it pretty much in one day although that was pre-Harry in March 2010. 3 days will be sufficient enough for the two even with Potter in there
I've always felt outside of the Magic Kingdom that both Universal parks compare very well to the other parks at Disney. Out of the two between Universal Studios and Islands of Adventure I always say if you like the rides then stick with Islands. If you like a nice blend of rides and shows you'll prefer Universal. Anyways, here are the must do's:
Universal Studios:
Revenge of the Mummy - A great coaster. You can literally feel the heat from the flames spraying at you. Pretty intense I think. Worth the price of admission.
E.T. - I've always said this ride is one of the few that captures Disney-like magic. Beautiful theming, fits right in there with the movie. The lineup alone gives you the realistic feeling you are in the movie
Hollywood Rip Ride Rockit - For the extreme roller coaster enthusiast. Comparable to a Cedar Point-like ride.
Twister - Some people found it underwhelming but I didn't. It won't scare you to death but it's worth it just for the theming.
Jaws - If you have a great fear of sharks don't do it because there are anxious moments. It could be a lot more intense, but enjoyable especially if you loved the movies
Simpsons - Worth it, a great simulator
Terminator 2 3-D - If only they did the 3rd movie just like this. It is truly wonderful, I think, don't miss it
Horror Make-Up show - You'll be in stitches, trust me
The rest is still good, but if you have to miss Animal Actors or Woody Woodpecker's coaster (similar to Magic Kingdom's Barnstormer) then so be it. Don't miss the bolded ones.
Islands of Adventure:
Incredible Hulk - It isn't uncommon for someone to say this is the best coaster they have ever been on. If you love coasters you can't miss it
Spiderman - Some simulators you know you are actually in a ride. I feel Spiderman does the opposite, very realistic
Dueling Dragons - If you love coasters go for it. I do, but I have had a fear of the leg dangling coasters all my life.
Jurassic Park River Adventure - Great ride with a steep and much longer drop than Splash Mountain. Suspenseful as well with the T-Rex on the loose
Ripsaw Falls - On the surface it looks like a mickey mouse ride. It isn't. You will get wet and the drop at the end almost feels like a 90 degree plunge. Either way, it is much more intense than Splash Mountain
Popeye's Rat Barges - If you don't want to get soaked then wear a poncho. This is a stern warning. The best white water raft ride of all time, nothing comes close.
There is more of course. If you have time try stuff out like Camp Jurassic which is fun with caves and enclosed tunnels made out of rope. Or give te High in the Sky Seuss ride a shot, it's a lot like TTA at Magic Kingdom. Either way you'll have a blast for sure.