The water parks are definitely worth it if you and your kids enjoy the water. I've been several times with all adult groups (30-somethings) as well as with my sister's kids (ranging from baby through 4 years).
As far as which park is better, I think it really depends on what your family enjoys. Blizzard Beach has more and longer slides while Typhoon Lagoon has a superior wave pool and the Shark Reef. The kiddie areas of each (which my niece has loved ages 2-4) both have their pros and cons. Probably more to do at BB, but it's easier to keep track of little ones and shadier at TL.