Northerner here, and Southern Pho is sweeter. Probably not noticeable to non-natives, but we always detect a slight sweetness in Southern food. Pho originated from the North, and I don't know anyone who actually comes from the North and prefers Southern pho. They exist, I'm sure, but while living abroad, the most common complaint with Northern Vietnamese is the lack of Northern-style pho.

Not sure about being inferior, but we like our broth to be clear. If you want heavy broth, you'll need to tell the restaurant to give you "fat broth". Maybe it started with us being poor, but we're not anymore. We want the broth flavor to come mostly from high-quality bone broth, not overly spiced. Same for the meat; most Hanoi Pho restaurants don't serve meatballs. We prefer high-quality rare beef over a random mix of beef. Northern Pho always comes with fresh, flat, and wide noodles. Outside of Vietnam, restaurants usually use narrower noodles, and usually not fresh. Sometimes they charge more for fresh noodles. It’s very common to eat Pho with a fried dough stick in the North. We dip them into the broth.