digital modulation
Since different schemes of modulation present trade off relating to each other, we should not analize digital modulation for a isolated point of view. For instance, if you analize bandwidth the M-PSK scheme decrease bandwidth as M increase, but this happens at the expense of degradation in error performance. If you take QAM you will see that for M>=8 it presents a better performance related to 8-PSK regarding power X error performance in spite of the fact that 8-PSK is generally used in sattelite communications since QAM is not suitable for non-linearities in the transmiter-channel-receiver system. Lets be concise, you must consider all the variables involved to decide which squeme is better suited for a given application.