Koraidon fits way better with the game's predetermined personality for your ride, especially with its idle animations. It also looks better when it's not in battle form, as
>>53444404 said.
Miraidon just looks weird, primarily since the vibrant yellows that usually define its active form are completely gone and just replaced with various shades of purple. Meanwhile Koraidon keeps all its colors intact and actually looks a little better because you can't see the retarded belly coloring and its lack of a headdress or cowlick makes it look less anthropomorphic (which is a good thing when talking about something that's supposed to be equivalent to your car).
Also, dare I say it- Koraidon running instead of actually cycling while being ridden makes it far more endearing. It's like your dog trying to walk on two legs- he's clearly got the capacity, and he's trying really hard, but he just can't do it and it makes you sympathetic.
