Whenever I see these comparison threads, so many people say that if you're seeking "thrill rides" go to Blizzard Beach. I find that a little misleading because BB has so many great mellow rides as well.
Our kids are 14, 10 and 6 and next month will be our third visit to BB. The kiddie area is fantastic...the "middle-of-the-road" rides are terrific...and yes, the thrill rides are great fun as well!
There's a wonderful ride where the whole family goes side-by-side down individual slides on a mat and you "race" down to see who's first. That, plus the big raft ride where the whole family gets in together are just wonderful.
One bit of advice I'd give is try to avoid the ski lift that takes you to the top of the mountain where the taller rides are. Yeah, it's fun going on the chair lift...but the wait can be very long. If you walk up the staircase behind the mountain, you can get in two rides for every one you'll get waiting for the lift. (But, I admit, especially with little ones, walking up those stairs can be tiring!)
Have fun whatever you choose....they both are among the very best waterparks in the country!