With a 10 year old, I'd say Blizzard Beach. But again, this is all a matter of personal prefernce as seen from the above posts.
Why do I choose BB over TL (we always do both as we can never decide!)?
1. BB has a great tween area (Ski patrol training area). Your 10 year old would love it and it rarely gets super crowded.
2. The family raft ride at BB is the best anywhere. Period! It is long and fun and enjoyed by 2 year olds to 75 year olds! There is no need to lug a heavy raft up a hill like at TL. The line moves very quickly compared to the raft ride at TL too!
3. The lazy river has lots of fun sight gags and an ice cave that our kids enjoy.
4. Tobaggans! When our kids were your son's age, they loved the toboggans. Again, the lines were never that long compared to the major raft rides/slides.
TL has a great Water Coaster. The snorkeling with the sharks is unique, but short and very cold water. It's a fun diversion.
The wave pool at TL is awesome, but it does have a tendancy to tear up feet and hands if you're timid.
The lazy river at TL is great for adults to relax, but our kids don't enjoy it as much as the one at BB.
There is no designated area for tweens at TL.
The lines for the slides at TL seemed shorter (except for their horrible family raft ride).
All in all, you can't go wrong with either choice. Why not show your son the different areas and let him decide.