I'd be interested to know exactly what the differences are. I've got a student who swears that he will only eat Pho that says it is Nam Dinh style, but he's from Nam Dinh. Next time I see him I'll ask what the difference is.

Pho Hanoi (phở Hà Nội) has coooked beef which we often call "phở chín" ("chín" means cooked). That's traditional style we talk a bout all the time.

Pho Nam Dinh (phở Nam Định) has fresh beef and they put "nước mắm" in it. That's why you may perceive an unpleasant smell. We call it "phở tái" ("tái" means half-done). You can see most people here in Hanoi eat this style of Pho. Unfortunately, most of Hanoians now aren't really Hanoian so they misunderstand that half-done pho is the traditional pho.

Some eat cooked and half-done meat together, we call it "tái-chín".

Nowadays, they change traditional pho to many types: ốc bò (gasteropod (not sure it's the right word) and beef), tái lăn, nạm gầu...

But I like Hanoi traditional pho (cooked beef pho) best!