Hi Matt:
Your family will love both Water Parks, Typhoon Lagoon and Blizzard Beach. My boys, ages 8 and 6, love both Water Parks. So while this question has two right answers, I'll give you my family's opinion.
My boys are thrill seekers, and they enjoyed Blizzard Beach a bit more for the thrill rides, especially Summit Plummet. I would poll your five year old. If your five year old doesn't enjoy thrill rides as much, then Typhoon Lagoon may be the preferable Park. Both Parks are large enough so that thrill seekers and those looking to relax should like both Parks. But if thrills are the name of the game, then I would look to Blizzard Beach. But the safer bet is Typhoon Lagoon, in my opinion.
To beat the crowds, get to the Water Parks at opening. It will help you claim the best lounge chairs and get you on a few rides with minimal wait.
Enjoy your vacation.