If the child is scared of dinosaurs, I think they will probably perceive the two rides as equally scary. I know when I was a child, it didn't matter to me, one dinosaur or ten. A dinosaur was scary and that was that.
However, there are a number of other aspects to the Dinosaur ride that may be the reason it is perceived as scary that have nothing to do with the dinosaurs. Dinosaur is dark, loud, moves faster, and is jerky. You also have the added storyline that an asteroid is going to hit and kill everyone. That alone might be scary enough. Then there is the not knowing what's ahead or around you because of the speed and darkness that can be scary. There is also a small drop at the end. As far as the dinosaurs themselves, the scary ones are large, in your face, and there are more of them, than there are in JP.
Jurassic Park on the other hand is half outside half inside, and though the inside is dark, it is still much better lit than Dinosaur. The large dinosaurs you see on the ride are all the friendly plant eating type. The scary man-eating dinosaurs are small raptors except for one T-Rex. The ride moves slower as you are in a boat. This could be a relief to some or scarier as you feel like there's no safe fast get away available. JP seems like it is more suspense driven scary. You see the damage the dinosaurs have done, you receive warnings of danger, but as far as big scary dinosaurs shoved in your face, not so much. Alot of the time, you don't even see any dinosaurs at all. There is also the long drop at the end to consider. Spoiler: There is one spot where a large box containing raptors comes down like it is going to fall on the boat and crush it. That could be scary.
I would say overall that Dinosaur is the scarier of the two because of the speed, darkness, number of large scary dinosaurs, loud noises, and the sense of impending doom.
