I think comparing the two films is a red herring because the two are vastly different movies due to their intended theme.
Thematically, the two films are incredibly different. Koe no Katachi is a story about Ishida owning his guilt and shame that he created through his actions as a grade schooler and, in the process of redeeming himself (learning sign language, later befriending Nishimiya), also grows into adulthood. At the climax of the film he no longer emotionally distances himself from other people (symbolized by the “X” on people’s faces falling off) and becomes capable of seeing everyone eye-to-eye, no guilt or shame weighing him down. The romance between Ishida and Nishimiya is simply a byproduct of growth as characters and the relationships they all shared within the span of the storyline.
Contrasting, where Koe no Katachi is driven by redemption, Kimi no Na wa is driven by romance. While Ishida in KnK is motivated by his guilt so as to make amends with Nishimiya, Mitsuha and Taki are motivated by their shared experiences through body-switching and, in the process, they develop romantic feelings for each other, because hey, what’s more intimate than sharing their literal lives with each other? And that love they develop for each other is what causes everything to tie neatly together through their red-string of fate, from Taki seeking out Mitsuha in the flesh, to the metaphysical – and almost spiritual – reunion across timelines.
So you really can’t compare the two films – as overall masterpieces at the very least – as they both tell two different types of stories. Visually speaking, though, is a different beast for another day. I will say this: Koe no Katachi is more visually appealing than Kimi no Na wa. And say what you will about Shinkai, at least that guy knows story structure.
Addendum: if you haven’t read the manga for Koe no Katachi, I would read it. The character motivations are more fleshed out and they are more three-dimensional than what is shown in the film.