Question: Why are people comparing these two movies?
Answer: Because they both contain horse races.
The way almost every single reviewer of Hidalgo mentions Seabiscuit makes me wonder if they've seen either film. Seabiscuit is a long, dull movie about a jockey, a trainer, and a horse owner who want to win horse races. The horse itself is only incidental. Hidalgo is the story of a cowboy and his horse. In this movie it's the race that's incidental. As Viggo Mortensen, who plays the cowboy Frank T. Hopkins, said so well in an interview here
I liked [Seabiscuit] and I think comparing movies is always a weird thing but since the name of the movie is the name of the horse (like this movie), I think in this story I do think you get to know the animal as an individual performer, as a character in a way that you didn't in Seabiscuit. They're different kinds of movies but the horse has a lot more personality. In Seabiscuit you're told that the horse has personality. You're told he's small and an underdog. It's other people talking about it. In this you can see Hidalgo's behavior, this horse.
I didn't like Seabiscuit much and thought Tobey Maguire's acting was very weak. Hidalgo, while not brilliant, was an enjoyable old-fashioned adventure that has a message that I think reviewers miss when they criticize how PC it is regarding the massacre of American Indians by government troops. The movie is not trying to make a political statement about American history but a personal one about Hopkins. He was half-Indian but had been denying it all his life until the message that he delivered helped spark the massacre and gave him a case of the guilts. It's a story about a man finally accepting who he is and doing something positive with his life because of it. That's just as inspiring as Seabiscuit's "little horse that could" tale.