Since the two resorts are "sister" resorts you have privileges at both resorts. The slide at FQ is probably better for younger kids and the one at Riverside for older kids. Same is true of the playgrounds. The arcade at Riverside is also bigger. And, Riverside has a sit down restaurant in addition to the food court. But the food court at FQ is brand new so it might be nice to check it out.
My son loves the pool at FQ. He likes all the alligators that are hanging around, the clam that cascades water into the pool. There is a kiddie pool close by too, which is nice. But, he likes the slide better at Riverside.
By staying at one we get the best of both. It is rather nice.