Incorrect... By themselves, their speed is no different from each other. I don't know where you got that from, they all run and move at the exact same speeds. The difference between Dixie, Diddy, and Cranky is based on difficulty. Just like in DKC2, Dixie is easier to play as. Diddy is still helpful, but I feel like his hover being more stable makes it easier to land on platforms (I actually used Diddy's jetpack for jumping on Lord Fredrick, he's best for that), and Diddy is also fastest underwater thanks to his rocket. Cranky is the technical one, and the more difficult buddy to play as, but I find him the most enjoyable. Bouncing through a stage as Cranky takes skill, but it feels oh so good. Plus, Cranky has an offensive advantage being the only character to jump on spiky enemies (which are pretty prevalent in the game).
By themselves, the buddy Kongs are no different than if you were playing as DK with them on your back. As far as gameplay mechanics go, DK + Diddy/Dixie/Cranky = Diddy/Dixie/Cranky. The difference is aesthetic, but it's still an enjoyable aesthetic that makes for a nice bonus at the end of the game.