I currently have both and I would neither call the M2s inferior nor worse in build quality.
The M2s are certainly different: lighter, more dynamic, slightly more portable (fully fold), very well built with real leather, less clamp force, and more versatile (bluetooth/ANC/usb dac).
I thought the M2s were more fun at first too yet I find myself listening almost exclusively to the PM-3s (for most genres, I do prefer them; find the M2 better with 90s r&b for instance). The PM-3's separation, clarity, and sheer smoothness is unmatched by any of my other current phones. They're still very portable and $100 less msrp than the M2.
I'd say if you need wireless, noise cancelling, or want a usb dac without carrying extra equipment, get the M2. If you just want impressive sound and portability, get the PM-3.