lol. Well it depends on what map, and how good you are. if its close range, id say machine gun. Or if the map is big, and has nice hiding places, go sniper.
erm depends on the range in my opinion. If your in a tower at high elevation or are almost completly hiddin its common scence to use a sniper rifle. When your eaither charging or at close range its common scense to use a machine gun =]
Most of the time, you aren't up high looking down on your enemies. They are near you so a machine gun is better since there are more situations which a machine gun is favorable over a sniper rifle. The sniper does have awesome range but I think is more effective in surprise attacks and ambushes.