Wattage at the coil and battery life. Dual coils divide the wattage between them. So if you are running a single coil at 8 watts, and put a dual coil on, you are running two coils at 4 watts each. This means each coil is producing less vapor and heat, but it's doing it in tandem so you experience the opposite, and you have the possibility of cranking it up as high as 16 watts(assuming your device can do so) and getting twice as much performance as your single coil, which if you cranked up to 16 watts is probably just going to burn up. Dual coils, however, require a lot more amperage to push that wattage, which will kill your batteries faster, and can even flat out kill low capacity batteries. Don't use them on anything under 1000mAh is a good rule of thumb, not if you want the device/battery to keep working.