Using variable watts more is more consistent than variable volts mode, but it isn't a panacea. Say a coil is either four wraps (1.8 ohms) or six wraps (2.7 ohms). If you use the same power for both of them, then that power will have to heat 50% more metal and 50% more eliquid in the case of the 2.7 ohm coil. As a result, the higher resistance coil will be cooler and the vaping experience will not be the same. On the other hand, both coils might be the same number of wraps but, to do that, the lower resistance coil must use thicker wire. In that case the lower resistance coil would have more mass and more surface area and it would be the cooler one, although the difference would be smaller.Use it in variable watts (power) mode - it does the calculating for you. All you have to do is click up or down to the setting you like.
I agree, it's easier to adjust using power rather than volts, and that is probably what a beginner should do. As others have said, start low and increase until you like the taste. But expect to have to change settings when anything changes, even if it's just a new coil with (theoretically) the same resistance.