Well, it depends. How important is atmosphere and theming to you at restaurants? I mean, both the Rainforest and T-Rex are spectacular in their ambiance and unlike anything else out there. They are so much fun. I get the argument that if you've done one, you've done the other, but that sorta is unfair. I mean, one has tropical creatures from a Rainforest interacting with you and the other has Dinosaurs. One has an Ice Age Room that turns colors, the other has a volcano. Each are amazing in their own right.
But I get the detractors and those that are not impressed as well. Both are sensory overload and noisy too, as the creatures are loud. You have to be in the mood for this sorta of themepark eatery. Also, the food at both is sorta at the level of a Fridays or Applebees or Chilis - that sorta thing. It's not fine dining. I think the food at T-Rex is a bit better. But if you're not picky and cool with burger and fried and other basic fried sorta things, it's very good. I've had the burger and a steak and been happy with those at T-Rex. I've had a pasta dish and a shrimp dish at Rainforest and didn't like it much. So, probably be aware that they aren't great with the more ambitious sort of dishes that require spices and seasonings and different sauces.
If you can only fit one in, that's fine, you will be more than pleased with the overall experience with the Animal Kingdom Rainforest Cafe. The one in Disney Springs is bigger but it doesn't really matter. For some people, this might be their favorite overall dining experience in Disney World, believe it or not. For my mom, it was, it was just so unexpected, all the animals interacting and the meteor showers, she found it so magical and was in awe - you could have asked her what she had to eat afterwards and she would not have remembered that but she would go on and on about how it was the best time ever and the best restaurant she ever went to. She loved it.
So, it really depends on your party and if you're sorta craving this sort of experience and how likely you are to be in awe of it or if it would do nothing for you. Everyone is different.