I would say, someplace outside the ring road, or failing that, just go anyplace that strikes your fancy. Here's why:
I usually don't completely agree with the dogma that you should avoid obvious tourist restaurants. Yes, the food will be somewhat dumbed-down to meet the general expectations of visitors. But it's probably decent food, and if tourists are eating there, it's a good guess that the restaurant has a good view. But if you specifically want good mussels, this is a different story. Despite it's reputation as being the quintessential Belgian meal, mussels usually have a very limited season in the country. In-season, you can almost guarantee luscious, juicy and tasty bivalves. Outside of the season, though, they tend to be small and somewhat stringy. That's the reason why restaurants that aren't catering to tourist expectations usually don't offer them out of season.
The season varies somewhat each year. September may or may not be the right time. In the tourist-heavy center of Brugge, it will be kind of hard to tell, because they often serve mussels all year, regardless of quality. If it's the right time of year, you can probably just pick the restaurant with the prettiest view. Mussels are a pretty basic, easy-to-prepare meal, so there's not likely to be much variation in quality.