I flew United monthly for about 2 years in 2011-2013 from Baltimore or Dulles and never had an issue. But these are heavily traveled routes. Are you flying non-stop or with connections?
The one thing I liked on United was having the option to pay for extra leg room seats which they don't have on Southwest. Depending on the plane and if you got an exit row, you had far more room than you get in first class.
I think the Dept. of Transportation keeps statistics by airline and airport so you may find some details about Duluth.