I asked this same thing for our upcoming trip, traveling with an 8 & 5 year old and was told almost unanimously - Blizzard Beach. The reasons I was given:
1. The wave pool at TL is very rough - the waves are rough and the bottom of the pool is rough, so if they get knocked over they could get pretty scraped up
2. The family raft ride is better at BB
3. The lazy river is longer at BB
4. The wave pool is much tamer at BB
5. The snorkeling is not great for young kids for a couple reasons. The first being the water is kept quite cool for the animals which can bother kids (and adults!) and also because the snorkel masks that you have to use, are spray treated with a very harsh chemical that tends to burn their skin and hurts/stings.
My own personal input just from looking them over, definitely BB, for all the reasons i was given, but also for two words: Tobaggan Racers. Seriously, check those things out -they look awesome!