These are the places where the duels with his former master take place. Both have similarities with Vader's thinking at the time as you can notice reading this: Mustafar is a hostile planet, but not completely inhabitable. Vader was a hostile and evil man, but there was believed that he still had goodness in him. Or so Padme prayed. And it was an active or-so-you-call-name-it planet. This also aludes to Vader and being in his prime, seemingly unstoppable. The Death Star has absolutely no use for the good of the galaxy. Vader is at that time "irredemable" as Obi Wan put it. He has absolutely no goodness in him at the time of ANH. Or so he would have us believe. Also, the Death Star HAD to be destroyed to save the worlds from an untimely fate. Like Vader (as well as the emporer) had to be killed for the Empire to end. Another thing, the DS was not a "biological" planet. It has no activity of a real planet or even moon. This also aludes to SUITED Vader and his being sort of clumsy inside that suit. A man-made moon cannot be compared to a natural moon. A droid-made black suited cyborg cannot be compared to the strongest Jedi.