I prefer to spin at a higher cadence because most of the time when im on a flat, i will burn out before i can get my heart rate up to my max, which i want. Going up a hill, my heart rate goes up farther and for a longer period of time when i keep 90-100 rpm up the hill in a lower gear, which means more time up the hill, therefore more time with my heart rate up, which means more calories and fat burned, etc.
If I stand on a hill, i will keep a higher gear with a cadence of around 45-50. Because I use my weight when standing like that, i can go faster, but it doesnt do anything for me fitness wise. I dont race, but i imagine that it will be faster to stand up a hill and a bit less taxing on your legs since you are using your weight to help you.