Pick one
The Eldarin languages remained remarkably fluid throughout Tolkien's life- he was constantly changing his mind. A lot of the problem is that in the 1951 revisions to the Quenta, Sindarin didn't yet exist. Thus there was no problem with Feanor giving Melkor a "Noldorin" epithet- although T still reckoned he should distinguish between Kornoldorin and the later Exilic form. By '58 the linguistic history had shifted, so that there was no way Feanor could have used (Sindarin) Morgoth. At any rate, T. changed his mind and changed the name to suit his own tastes or his current phonological theories. I suppose by some peoples' lights the later form is more 'correct.' Or one could, very artificially, decide that one represented ancient Tunalambe and the other Exilic Quenya.