The 7.4 disc would be my choice for the urban streets of Detroit. Wider tires, more robust rolling gear, and still good componentry, and more gear choices for the varying speed choices you will be stuck with. More than the 7.4, but still way less than a 7.5. When you wear out those tires, I'd even consider going slightly wider for more comfort and capacity. Wider is better on bad surfaces.
Changing from a box store mountain bike, the change will be amazing. I think that the 7.4 is the sweetest spot in the Trek lineup in value for your dollar. The extra money will go toward things like rack, panniers, lights, bell, bar ends, etc.
MHO