I do love em dashes, but one of the nice things about commas is that they are basically invisible. So if I don't need to call attention to a detail, sometimes a comma is better because it doesn't distract the reader. Because an em dash draws the readers' attention, they can be used effectively for impact—or to help someone remember something 12 chapters later.

Out of context, I think it's difficult to punctuate the queried sentence. If the sentences surrounding the queried example were talking about other wands of various lengths and makes, I might use em dashes to call attention to specifically which wand I meant.