[Luthien_ Tinuviel]

Lord of Angmar is most likely correct. To add to that note, and if you go in for grammar at all (I don't, really, so I'm not quite sure why I'm saying this, but bear with me), although they are both adjectives, Elvish is usually used to describe someone or something's attributes (her manner of speech was distinctly elvish, or the ring is elvish in origin, though where it comes from I know not), whereas Elven seems to be used primarily as an identifier (the elven-blade, or down the stream they rode in their elven-boats). Overall, however, the terms seem to be interchangeable, with Elven being the favored term.