If it doesn't say Wisconsin or Iowa on the jersey, I'm cheering for Big 10 teams in the non-conference 99 times out of 100. But this is the one exception.
It's going to be a tough year for Northwestern, and if they somehow beat Butler, it diminishes a Gopher victory should they beat Butler, especially in terms of public perception. It's tough to argue that beating a good Butler team (one that handled Northwestern with ease) wouldn't look better on the NCAA resume than beating Northwestern twice, despite Northwestern beating Butler.
While you could argue that the Northwestern win might buoy their RPI, making those games better for the Gophers in conference play, I think Northwestern will lose enough games this year that the minimal game to their RPI, taken from the Minnesota perspective, is outweighed by what it would mean to beat Butler and be one of the few teams to beat them this season.