Both lazy rivers are nice. But if I were going without the kids I would choose TL. It seems a little more tropical, adult, relaxing feel to it. Blizzard Beach is probably a little more funny themed.
For instance in the TL lazy river there are lots of plants, misters, you feel like you are floating in the tropics.
At BB there are still a lot of plants, but there are funny little things to laugh at along the way (a snow cabin in middle of river that sneezes).
Both are lots of fun though and I would recommend either one. TL just seems a little more "adult" and perhaps more romantic. If you can call swimming with 20,000 people romantic!