Euronymous's fits the phonetic realization better (e.g. [ju'ɹɔnɪməsəz]) but either is stylistically acceptable by generalized rules of English orthography.
Ha, yeah, that reminds me of: when I went to see Cynic (and Dragonforce hahahahahahaha) I was leaving the venue and happened to notice a Kid Rock poster on the wall. He dated it 99'.