Blizzard Beach, I think, is a bit bigger, with more elaborate slides (and some that offer genuine hair-raising thrills.)
On the other hand, Typhoon Lagoon offers a surfing pool, with humongous waves and the Crusher Gusher -- a roller-coaster-style water slide that actually goes up hill and down. But many of its slides are shorter and tamer than those at BB.
Wherever you go, go EARLY. If you are staying on property, take advantage of the extra hour they offer youin the morning. These parks fill up quickly and by noon, it's a loooong wait for some slides. Or worse: BB often fills up and they stop letting people in.