Both headphones offer a good level of sound quality. With the M30x you do get a slightly more powerful frequency range of 15-22k Hz when compared to the M20x where you get a range of 15-20k Hz. We gave you a visual chart above with this.
There is also a difference in power handling with 700 mW on the M20x compared to the 1300 mW on the M30x.
An area where both headphones don’t excel is in isolation. They are quite average in isolating the sound but this is to be expected with headphones at this price point. It’s definitely not the worst out there but it’s an important point to consider.
Where both do excel is in the bass as this is punchy and offers great depth. You’ll love being able to listen to the rich depths of sound with bass-heavy music. The mid accuracy is also good but both can be a little lacking in the treble.