We are a Disney family, but just spent three days last week at Universal without doing Disney at all.
Personally I would not break up my WDW trip for a day trip to Island of Adventure for that cost. We did it separate for that reason. Since we had never been I truly didn't know what my family would like best, and I'd have to say, IOA we spent double the time in over Universal.
It was also nice to try and immerse ourselves into the Universal Orlando atmosphere. We stayed at Hard Rock, and had three days plus Halloween Horror night for the adults.
I can say we enjoyed ourselves, and some of the rides were fantastic, but we still prefer Disney. Standout rides for us were Forbidden Journey, Hulk, Spiderman, The Simpsons, Dueling Dragons, and Despicable Me. To be honest, I've never been on a ride comparable to Forbidden Journey.
Good luck!