Joe Biden is senile... he cant control his mouth or his mind, so random gibberish comes out that doesnt even make sense in terms of the english language.
By contrast Trump isnt Senile, he is just a severely challenged moron. While he has motor control over his mouth and some control over his mind, he is just a moron and the things he says may be grammatically correct, but logically are just bonkers.
They are both mentally unfit to be int he workforce at all, let alone as a president. But both of them are unfit mentally in very different ways.