Such a hard vote!
Typhoon Lagoon definitely wins in terms of atmosphere/theming and the wave pool. It's always seemed less crowded to us, with the longest lines around the 45 min mark, but several of the body slides (i.e. no raft) around the 20 min mark by mid-day in the summer. When the lines seem like too much, there's always the wave pool or hanging out in the shade enjoying the tropical music. On the downside, there's a lot of carrying your raft up the stairs and there are fewer slides. There is a kiddie area, but unlike Blizzard Beach, there is not really an area for the kids aged 5-10.
Blizzard Beach has many more slides (two of them are intense), you only have to carry rafts up part of the way, and the kid's area is great. On the downside, once it's crowded, it's just plain crowded. Lines for many of the rides (non-weekend, summer) can run anywhere from 40 mins to 90 mins (summit plummet) and the pool is smaller and also gets seriously crowded. Blizzard Beach is awesome first thing in the morning (particularly if they have EMH), but gets tough to manage by 12:00.
Our group of 5 kids (10 - 16) like both the parks a lot and it's also hard for them to choose their favorite!