Well, it really depends on what you prefer. If you go for the slides, Blizzard Beach is your choice. There are more, better, and more thrilling slides at Blizzard Beach than at Typhoon Lagoon. Typhoon Lagoon has good slides, but not as many as BB. TL has a surf pool with huge waves, which BB doesn't have.
Typhoon Lagoon has a tropical theme, while Blizzard Beach looks like a ski resort.
Bottom line, if you go for slides, Blizzard Beach should be your choice. They're both great, however.