I don't know about 15 year olds but my 5 year old son remembers T-Rex as his favorite thing about our entire trip. The food is what you'd expect (okay, not great) but the theming is pretty excellent.
My 15 yr old daughter loves them both but gives the edge to t-Rex. The food is similar at both. One has jungle animals and some aquariums in it and the other dinosaurs. Rainforest has periodic rain showers where the animals make noise and move. T-Rex has periodic meteor showers where the dinosaurs roar and move a bit. Just depends on which theme interests him most.
T-REX... is a favorite of ours, and just edge's out RainForest.... each spot is fun... the menus are about the same, good choices, pub grub style food.
Actually you could do both, when you are at AK you could do Rainforest -they serve breakfast, lunch and dinner (only at this location breakfast is served), you can access it from outside the park and inside the park as well, We have had breakfast here is a nice way to start the day...
Then another night you could check out Disney Springs, and go to T-REX...
I have never been to T-Rex but frankly wouldn't waste my time or money at Rainforest Café. Food is mediocre at best and the theming is formulaic and pedestrian.
I vote for T-Rex too: I like the theme-ing and while the food may not be the best, it's still pretty good and miles better than Rainforest. Also, we have Rainforest at home so like someone else said, T-Rex still feels "special". Don't forget to look for the hidden mickey near the octopus!
These are Landry's restaurants, are they not? You can join the Landry's VIP (or whatever they call it) and they'll give you priority seating without a reservation.
Disclaimer: I bought the VIP membership but never got around to using it. The reports here have been that the seating-without-reservation worked really well.