You can look at a crowd planner website to see what crowd levels to expect. We were there for July 4, and we were very surprised that the crowds were light compare to the days before and after the holiday itself. That said, you should look at which activities your family would enjoy to decide between Universal Studios or Islands of Adventure. If you are Marvel fans, the new section at Islands of Adventure is really cool. My 13 yr old enjoyed the Simpsons land, while my 6 yr old enjoyed Dr. Seuss land.
You do not have to purchase Express Pass in order to enjoy the parks. My 13 yr old and I were there recently, and we just went in the Single Rider line if we didn't want to wait. Single rider is especially helpful for any virtual rides, since you don't really share the experience anyway.
I would definitely not spend the money to purchase Express Pass. I would, however, consider upgrading your single park tickets to park-to-park if you can't decide between the two parks. I know you aren't Harry Potter fans, but the Harry Potter worlds in the two parks are really fun to walk around, even if you aren't going to go on the rides. Taking the Hogwart's Express train between the parks is fun. The HP experiences are the most "Disney-like-with a twist" things you'll do at Universal. Being a fellow Disney fan, I'd encourage you to go experience it. Full disclosure, though - I've read all the HP books ahead of my older child, who has now read them each at least 3 times. I prefer Disney, but she has convinced us to enjoy Universal with her!
We've stayed at one of the Universal resorts with free Express Pass as well as off-site with no Express Pass. I can say that my young child and I did not miss having the pass, but he doesn't ride roller coasters. My husband and my older child did use it when they had it, but they also enjoyed the rides by doing single rider on our other visit.
I can also say that we didn't have any meltdowns - the parks are doable. Try to arrive early and do your top picks. For example, we are not Transformers fans, so we skip that attraction every time. But, we do enjoy stopping for the street entertainers and enjoying the scenery. My little one would spend an hour interacting with the talking Mystic Fountain if we let him. On balance, I'd probably pick Islands of Adventure, but my 13 yr old would probably go with Universal Studios. It's why we need the Park to Park!
I hope you enjoy the day and get a taste for the parks whichever you choose. Think of it as a preview to decide if you want to take more days at Universal on your next Orlando vacation, and possibly stay at one of the 3 Universal resorts with free Express Pass. That's what happened to us! We're even splurging on club level next trip. Gasp!
