Pyongyang naengmyeon noodles are made from buckwheat. Hamheung naengmyeon noodles are made of potato or sweet potato starch, which make them especially chewy and rubbery. Servers at restaurants will usually snip the noodles with kitchen shears at the table, unless you have a perverse delight in trying to consume a neverending tangle of noodles in one go. I've discovered that most people's preference for one style over the other usually depends on the texture of the noodles.
I usually order the mul naengmyeon over the bibim in whatever city style. I'm a soup person. But since bibim naengmyeon is Hamheung's original, I went for the bibim this time.
Sliced pickled mu as a side dish, and if the spiciness isn't enough for you, you can add hot mustard to up the tongue-burn factor. (Korean mustard isn't like the western mustard; think of it as a yellow wasabi.)