Ouranos is the Greek spelling. As far as the Greek god is concerned, that is the better name.
Uranus is the Latin spelling. Just like the other planets planets are named Mercury not Hermes, Mars not Ares, Jupiter not Zeus, astronomers looked to Latin not Greek for naming.
However the person that named Uranus fucked up because while Uranus is the Latin spelling of Ouranos, the Romans called the God Caelus. Rather than use the Roman name (again, like Neptune instead of Poseidon), the person who named it just used the transliteration not the actual nomenclature.