i'm actually trying to make the same decision. i bought the gm because i love the bag but returned it because it is a bit loud for me. now i'm really sad i let it go but i'm also falling in love with the duomo. lol. this wasn't helpful.
i love both bags, but definitely the manhattan is dressier and a more lovely bag to hold. The duomo is subtle and good for a non-worry work bag...what will you be using it for?