We went to BB last week, so I remember it pretty well. Went to TL four years ago, so my memory is not as fresh. Having said that...
The main draws to TL are the reef and the wave pool. If you've snorkeled in a real reef, the TL reef won't be that big of a deal. It is fun, though. As I recall, the TL surroundings were very tropical looking, very pretty. Slides were fun, but I don't recall anything fear-inducing.
Blizzard Beach is more whimsical. The ski lift is fun, and offers a panoramic view of the Orlando area from the top. The slides are fun, varying from easy to "daredevil". Blizzard Beach has two areas for kids: one geared towards toddlers, the second geared towards the elementary school set. Having a five year-old, we spent a lot of time in the latter area. He really enjoyed it. The area has tame slides, a zip line, and the normal obstacle course type things you find at water parks.
I don't think you can go wrong with either one.
Oh, if your son likes putt-putt, Disney has a "Summer Winterland" putt-putt course adjacent to Blizzard Beach. My five year-old loves putt-putt, so really enjoyed it.