I would want to be a human in that time and place. I know a human's strengths and limitations. I cannot say that the same is true for the other species, but I will try (at least from what is present in the series...)
Sauropod: Big, unwieldy creature that needs great amounts of food and water just to stay alive. I imagine that the majority of my waking hours would be spent eating and drinking water. Not exactly the kind of life for me... there's more to a good life than consumption...
Triceratops: Probably the most attractive quadrupedal option. They needed to eat a lot I am sure, but not nearly as much as the Sauropods. There might be violence issues though, if the hypothesis that these creatures were aggressive proves correct.
Pterosaur: Flight would be great, but the creatures would probably be too delicate and small to be of much use in most endeavors. And the larger ones might be too awkward...
Hadrosaur/Anatosaur ("swimmers): Probably the best bipedal option...they were adapted for swimming, but I don't know if they were well-designed for long treks over land. Ducky, for example, often rides on Spike's back... her feet were best designed for swimming, not walking, I suppose... these creatures would have to spend much time in/near water or on marshy terrain.
Theropod: Mostly banned in the valley, must eat meat to survive, and even if young ones can be tolerated, they will grow up...
Stegosaurus: Large and dumb. Not a good option.
Oviraptor: Good at sprinting, but unless I want to take flight at every danger, it isn't really the best option.
So if I could go back in time and go to the fictional place of the Great Valley, I would choose to stay a human.