GLONASS VS GPS
Due to less number of satellites GLONASS has some coverage issues in Southern parts of the world. While GPS has some coverage issues and blind spots in Cities with High Skyscrapers (apparently blocks the GPS Signal).
GLONASS does have some slight accuracy edge over GPS with 2metres of accuracy (10 meters for GPS). Using GPS + GLONASS allows you to be pinpointed with an unmatched accuracy since now you are accessing 55 satellites all across the globe. So when you are in a place where GPS signals are obscured like between huge skyscrapers or tunnels, you will be tracked by GLONASS satellites accurately.