With just 5 days I'd probably just hit WDW and skip USF/IOA, especially if I knew I'd be going back to Orlando in the future. Keep in mind I love USF/IOA, not as much as WDW, but pretty close.
But if your kids and you and your hubby/wife love thrill rides, it might be worthwhile heading over to USF/IOA. Keep in mind there is no way you'd be able to cover both parks in one day. I'd look at their website and look at the rides offered and pick one or the other. With the ages of your kids I'd take a guess and say you'll probably enjoy IOA a lot better, more for a variety of ages. With a 6 year old USF might be too much, they have some stiff height requirements and rides like Jaws, King Kong, Twister, and Earthquake might be really scary for him/her.
Like a previous poster said if you know you'll be heading back to Orlando in the future, it would be ideal to spend 2 or 3 days at a USF hotel (you'll get front of the line access) and then head to a WDW hotel and do that for the rest of your trip.
If you do go and want to try to tackle both parks in one day, you might want to look into getting a VIP pass. Last year they offered VIP passes, I think that's what it was called, it was extra $$$, but that would help you get front of the line access for the major rides. You have to get them VERY VERY early though, they sell out really fast. We got there about 10am and by noon they were gone.
IOA/USF have some pretty unique rides, but they are more of the thrill type. We'll be in Orlando for 9 days this trip and we're just staying at WDW, because of our youngest, she wouldn't be able to ride most of the rides at USF/IOA.
Good luck deciding and have a great trip!!
Pammy