Depends on what you think they might like better -- jungle animals or dinosaurs. We've been to both. DS3 was a bit scared in RainForest, less scared in TRex.
They are both very loud and active. RainForest is a bit darker inside, very cool with elephants and gorillas. Every 5 minutes the gorillas and elephants roar and shake their trees (the gorillas are what scared my son). TRex has several themed rooms with lots of large dinosaurs and mammoths. The volcano erupts and a meteor shower roars overhead.
So for my 2 boys, ages 3 and 6, they'd pick TRex first.