Why not do both? If you buy a two day pass (the unused days never expire) you can hop between both parks. But, if you really can only do one, it would depend on what type rides you like. USF is where you "ride the movies". They have simulated rides and rides that put you into the movies. No thrill rides. At IoA, the theming is wonderful and they have more thrill rides. They have 2 coasters and two flume rides, a rapids ride, a 3-D ride, etc. For me, I would do IoA if I could only do one. I would suggest you get on universal's web site and read about the rides both parks offer and see which park best fits your liking
Been there, done that, going back!!!