I've stayed at both resorts (actually, stayed at both 6 months ago) and they both are great resorts! At the Contemporary, I've only stayed in the Tower, on the Club Level. This is on the 12th floor, and offers limited concierge services (it is a few dollars a night less than a lagoon view at the GF). We were on the Magic Kingdom side, and the view was awesome. At the Grand Floridian, we've done both the garden view and the lagoon view. I like both views, though I think garden view is probably a better value. I should add that lately I've been booking lagoon view! There is just something about being able to sit on your balcony and stare over at Cinderella's Castle (make sure you request a MK view!).
It had been my dream to stay at the Contemporary since 1994, and finally got to check-into the resort in December 2001 (7 years later). Was I disappointed (after staying at the WL, GF, PR, ASMu)? Nope! This was an awesome place! It was Walt's original! Did I look over to the GF the entire time and say "wouldn't it be nice to be over there?"? Well, kinda. But, I knew in a couple days I would be over "there".
The Grand Floridian is Disney's flagship resort, and it is the flagship for a reason. It has excellent, beautiful, large accommodations. A great staff. Wonderful restaurants. Beautiful public areas. Great gift shops. Yes, there are things about the GF that I could improve (I'll let them be known if I could find a comment card!).
Enjoy your trip! Choose whichever resort you think you and your son would prefer.