As has been pointed out, there is no wrong decision here.
That said, I'd go with Blizzard Beach. It's the newer of the 2 parks and in some cases you can see where Disney improved on some things. The lazy river is longer at BB. The family raft ride is MUCH longer at BB. I also prefer the theme. If also has both a preteen section and a kiddie section, (TL only has the kiddie section). Also, there are the carpet golf courses nearby.
However, if you are interested in waves, the wave pool is much bigger at TL, as are the waves themselves. They also have the shark reef and Crusher Gusher 'water coaster' if those interest you.
I personally prefer the longer slides, including the big one, Summit Plummet. (which makes Dumbo look like Ellen's Energy Adventure in terms of wait time to ride length, far and away the worst in all of WDW. Wait for an hour, ride for 8 seconds, including the ride out at the bottom).
Enjoy!