"Chow Mein" is not even proper Cantonese. It's the Anglicization of a Taishanese word, and the dish as we eat it in the US today does not resemble Southeastern Chinese food. Given that I generally eat (authentic, home-made) Cantonese food around a few days a week, I'm guessing I have a pretty good idea as to what it tastes like (though, I will admit, they'll sometimes make Hunanese or Szechuan food, too, to spice things up a bit).
"Chao Mian", on the other hand, is simply a Chinese word for ANY stir-fried dish containing noodles.
Moreover, there's actually an author who has claimed that this specific dish (in its current, well-known variety) was invented in the US.
(BTW, evidently, in the East [Coast], Chow Mein is usually served with crispy noodles. In the West, it is served with soft noodles.)
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