We usually try to go to both, but our favorite is Typhoon Lagoon. It really depends what you like - I like the theming better at TL and the wave pool, but Blizzard Beach has more/bigger slides.
The wave pool at TL is really a surf pool; a large wave comes about every 3 minutes and wipes out everything in its path. It's difficult to be in this pool with small children, unless you stand near the shore, because they will get knocked down. Also, TL has Shark Reef and the new water coaster, Crush N' Gusher, which we haven't been on yet. They also have some good slides and a good family raft ride, but the family ride isn't as long as the one at BB.
At BB, the wave pool is small, continuous bobbing waves (which make me a little sick to my stomach after awhile), but is probably better for smaller children. There are no large waves here. But they do seem to have more, faster slides for the adults. Also, they have a chair lift that takes you up to the top of most of the slides, so there's not as much stair climbing.
Both parks have nice kiddie areas. My daughter is 5 and she enjoys both parks, but I would say her favorite is TL.
They're both nice and you can't really go wrong with either one - it just depends on the ages of your family and what you like.