They both work, but I prefer as. 'For' is more frequently used as a function word that could infer the 'wife' as passive, whereas 'as' is more frequently used in a manner of showing a relationship to something, putting the 'he' and the 'wife' on a more equal footing. Or you could avoid the conundrum by saying he wants to marry her.
How they sound depends on the context, including the sound and rythm of the surrounding words.And they may not mean what you intend, depending on that context. Examples: "Why does he want that little puppy?" "He wants her for his wife." "What part does Sean want Angelina to play in the movie?" "He wants her as his wife." "He wants her to be his wife" or "He wants to marry her" are possible alternatives to consider, depending on your needs. Of course, if you're doing poetry, choose what sounds right to you and don't worry about it.
