We did both during a 1 week trip, which was our first trip so we wanted to try everything. Did two separate days, and afternoon visits after the crowds were thinning out. Overall we were disappointed in the slides at TL, but loved the shark reef and the lazy river were very good and the theming is very good - a beautiful park. We fell in love with Blizzard Beach. Overall, BB is probably my favorite park at WDW - we have never missed it since, and we have not gone back to TL since. But, this trip we now have a 15 month-old, so we probably going to just to TL this year, because it has a better "kiddie" area and is overall more mellow. Also they have the new ride that I want to try.
This is a very personal decision - don't go by what others have done. Some families, as have posted, love the water parks above even the "main" parks. Other poeple can really take or leave water parks. I agree that they take a lot of energy. I would not try to do both water parks in one day. Just too much, and you lose a lot of valuable time in transit (closing up camp, turning in locker key and changing to leave one park, getting to the other park, and then re-renting a locker, setting up, getting ready, etc.).