I wanted to vote for both!
They are both amazing. Last Sept 2 wk vacation: I went to each twice!
While similar, there's lots of differences.
The big pool is a major attraction at each.
--Blizzard Beach has a steady rise&fall wave action and you can have inner tubes... it was so nice to be in the tube and just ride up and down.
--Typhoon Lagoon's would send out a huge wave every so often that would always knock you down if you didn't swim with it... and boy! You sure could go far! But if the wave wasn't happening, it was just a big pool with people standing around waiting!
I also found that there were more gentle-r rides at Blizzard Beach (and wild ones as well).
And the family inner tube ride was waaaayyyy longer at Blizzard Beach. And because the tubes were bigger, they held more people and so the line went much faster.
Blizzard Beach also had the Ski Lift, and after running up the stairs about 20 times, that sure came in handy.. Try the single rider line if your son is okay with that.
BUT at Typhoon Lagoon they had the Shark Reef...aka Swim with the Fishies! That was really nice! And once I found the Crusher Gusher, Whoosh! I was with family for that one (didn't find it the first visit...it's off to the side). Must have done that sucker 6 times in a row! As a triple and as a double! Word of advice on that one: try to keep your body straight or else each time you hit a bump, it's your butt that will feel it!
Enjoy!