Grandmas is more of a big time race with big expo, more runners and more support. It's a better race experience.
Minneapolis is more of a smaller local race. The logistics for Minneapolis is way better. It's also well run but not the main event in town which is Twin Cities in Oct.
To run Duluth you need to fly to MSP rent a car and drive to Duluth. Hotels in Duluth are expensive (they make you pay for three days)marathon weekend and hard to find. The race is one way so you need to get up early for a bus ride to the start.
Since the Minneapolis race is smaller and the city is bigger, Hotel space is not a problem and there are plenty of rooms walking distance from the start.