The PSB and the NAD are, as others have said, comparable in sound and both have a realism that makes them superior to others at the price point including Momentum and, at a higher point, P7. It is difficult to pin down why this should be but the loudspeaker reference that Barton uses has certainly paid off. The amplifier built into the PSB M4U2 is a definite bonus and has a similar effect to a Fiio. There is a sense of acoustic and texture which is quite striking when first heard. Either would be my go to phones.
They are both very comfortable with similar ear pads but neither would win prizes for svelte appearance. The PSBs look like Robo Cop and the NADs have an odd squareness across the headband. However both are comfortable for long periods.The NADs are appreciably lighter. Both fold , in different ways, and are reasonably compact in their respective cases. The folding mechanism on the PSBs, a complex metal hinge, seems very robust but may be vulnerable over time. It does make a much smaller shape when folded. The NAD capsules swivel flat but thats it.
The noise cancelling on the PSBs certainly works but produces, to my ears, an inferior slightly nasal sound. Its very easy to compare by flicking the switch between passive, active and noise cancelling.
I have not heard the PSB M4U1 s but assume they are the same as passive 2s but lighter.
I am glad I have both but if forced to choose i would have the PSB for active mode and, when required, noise cancelling. For a conventional headphone the NAD is definitely the king in its price range and beyond. Between the NAD and the PSB1s I would choose the NAD as lighter and less complex in its folding mechanism.