For just the slide, I would think the serpent slide at POFQ. However, if your 11 year old is a girl, she might like the overall theme of CBR better (lots of color).
We've stayed at all the moderates with our 9 year old daughter. The best slide by far is the one at CS. I'd put them in this order (for older children):
Keep in mind that if you stay at FQ or Riverside you can use the pools at either resort. The slide at Riverslide is a bit more windy and faster. The one at POFQ is longer, I think, but straighter without as many turns. Both are nice.
We stayed at POR in the mansions but used the pool at FQPO and my kids had so much fun, they are ages 16, 10,8,4..They really loved this pool, it wasn't as crowded at the other on at riverside ,
Mrsmom
I would go with PO-Fq, for the simple matter of then you can have the choice of 2 themed pools, and 2 slides! Who could ask for more? Have a great time!