I definitely think that Island of Adventure will be more fun if you only have one day and if your kids like thrill rides. UF is still great fun so if there is any way you can squeeze in both that would be the way to go, especially since you would still have four days at Disney.
As far as IOA goes, my 7 and 10 yr olds loved the Spiderman ride, I did too and although it was wild it wasn't a scary coaster type ride. A couple of the rides around toon town are great fun but you get VERY wet. Ripsaw falls is alot like a souped up Disney's Splash Mountain. The other one was the Popeye Bilge Barge (or something to that effect). It is a raft ride like the Disney one (Kali River Rapids ???) at Animal Kingdom, but again a little more exciting. Jurrasic Park was great fun but again you get wet. The big coasters, Hulk and Dueling Dragons are like nothing I ever saw before! Unbelievable! I couldn't go on either of them (although I wanted to) but my ten year old daughter did and found them awesome. There is a bouncy drop ride something like Disney's Tower of Terror, but it is outside. I did ride it with my daughter, I loved it, she was scared! My favorite part was Seuss Landing. My kids had a great time and there were lots of teens there even though it seems geared to younger kids (merry go round, dumbo type ride and a great Cat in the Hat story ride).
USF is alot of fun, the Men in Black ride is probably everyone's favorite.
The biggest pitch for Universal is the front of the line access if you stay in an on-site hotel! Disney can't come close for convenience. The best part would be that you only have to stay on-site for one nite for two days of front of the line access so you could do both parks! Universal ticket prices are also very enticing with many opportunities to get a bonus day. We'll be getting Universal Annual passes for the price of two days at Disney parks! I could go on and on.
Just in case you are interested my family and I were confirmed Disneyholics, over twenty trips in the past ten years and we've stayed at all the best Disney resorts. Most of our Disney trips were at least 5-7 days, our last one was last Thanksgiving and we stayed at the Beach Club. Our very first trip to Universal / IOA was a last minute junket this past Aug 26-31st just before school started. We had free plane tickets so we went on a weeks notice. Since we had never been to Universal we thought it was worth a try and we were awed!!
It was really that good. Maybe it was the ages of my kids but everything seemed just a little more exciting, the hotels were a little newer and nicer, the hotel staff was very friendly, the restaurants had better food, and there were more options and terms of price. Citywalk was great for food variety. The pools were super nice and best of all, the trip cost significantly less when compared item for item. My husband and I are going back in a week and a half to the Portofino Bay for just three nights (more free tickets) and then the whole family is going back over New Years (12/29-1/5). I'm sure we will go back to Disney one of these days but for now Universal and the on-site hotels seem more appealing.
Just my two cents.
