Is it simply a matter of preference? The em-dash can be used for any sort of interruption, and unless I'm unaware of some rule, I think this qualifies as one. Oftentimes I prefer the use of the em-dash in situations like this because I'm never quite sure whether a comma really gets the job done, or if it is technically proper. The em-dash offers a more abrupt break than the comma, which aligns better with how I want the sentence to sound, influenced by how I read. As for the technical aspect, I know there isn't really a good argument for why I feel that the use of a comma here may be improper—this isn't a comma splice. It's simply an interjection, an additional thought that is relevant to the meaning of the sentence, but not an essential piece of the sentence. I think of it as a flower in a woman's hair—the flower isn't essential, her hair is usually beautiful in and of itself, but the flower adds to the overall effect. At the risk of extending this metaphor too far, maybe this debate is as pointless as a debate about how to attach the flower—bobby pin or the stem alone?