Xerath may be magic incarnate, but it still pales in comparison to the star-forging process that Aurelion Sol is capable of, and it's a power that Xerath will never be able to utilize.
Aurelion Sol mentions in his VO that "love is as real a force as gravity."
He also mentions that the truest magic is unspoken, because writing, and speech by extension, merely butchers the intent.
The magic that Xerath learned was not as powerful as it could be; he learned the spells through written word, thus the intent is butchered, and even if he were to have the spell in his mind, he is merely reciting written words in his head. It is not the truest magic that Aurelion Sol talks about.
Now, take into account that Aurelion Sol forges stars, and is implied to be the only one capable of doing so, simply because he was brought into existence with that purpose. But to fulfill that purpose, he needs a singular condition, and for him, it is the unconditional, exceptional, purest love that he holds for his stars, because he places a tiny piece of himself in every star he creates. He loves his stars to a degree that we as mortals cannot fathom.
Each star is essentially, a near limitless fusion reactor. The tiny stars orbiting him were enough to permanently scar the land and caused an explosion that closed a rift in reality.
Aurelion Sol possesses greater power, simply because he is capable of using truest magic. While Xerath may be 'magic incarnate,' his utilized magic isn't as strong as it could be, because more often than not he would be drawing it from a "butchered intent."
He'd probably have some killer lightning shows, though.