Dawn,
I visited both water parks last Friday with this exact question in mind. My conclusion is that both parks are great for toddlers, teenagers and adults, but
Blizzard Beach (BB) is the better option for kids between toddler and teenage years. For this pre-teen group, BB's Ski Patrol Training Camp is just perfect, and there isn't anything very similar at
Typhoon Lagoon (TL).
BB's Ski Patrol Training Camp area has a very cool zip-line that drops kids into the water (I would have done it, if they had let me!), an iceberg walk, and some fun but not-too-scary water slides. I know my six-year-old boy would absolutely love this area (hope to take him on our next trip!), but would be bored by the "kiddie" areas at either water park (Tike's Peak at BB, and Ketchakiddee Creek at TL).
If you do go to BB, be sure to also try the Teamboat Springs family raft ride. It has some rough water at spots, but I think most brave toddlers and certainly pre-teens would absolutely love the ride -- and you all get to enjoy it together as a family.
Just to be complete, I should mention that your two boys
could have fun at either park (there are slides/pools/etc. for a pre-teen to enjoy at either), but since you asked for the park that is "best suited", I really think the answer is Blizzard Beach.
Last thing to mention, and I mention it because I saw a very very sad boy get rejected from going down a slide: please be sure that you avoid swim attire with any rivets, buckles, zippers or any other exposed metal (it can scratch the slides, so it isn't allowed).
Hope you have a great time at the water parks - whichever you choose!