I own both, APP and APM. Less difference than I hoped, especially given a $550 price for a semi-disposable gadget with sealed in batteries that will eventually degrade and need replacements.
I compared them using iOS's multiple headphone support, rapidly swapping between the two listening to Apple Music across many genres of music (radiohead, oscar peterson, andy shauf, max richter, jamie xx, the list goes on) .
The sound signature is almost identical, with the exception of much stronger (sometimes annoyingly sibilant for me) treble on the APMs than the APPs and a little more bass at the very low end.
That's not to say there isn't a difference. Over-ear headphones provide more tactile sensation on your skin and allow sound to enter naturally rather than being piped into your ear canals. The APMs do sound more lively as a result, in a variety of ways.
The APPs on the other hand get remarkably close, and you can barely even tell you're wearing them. Getting the APMs made me appreciate the APPs even more for the stunning masterpieces they are.
This matches my experience as well. The APPs simulate my outer ear sound colouration quite well, they are obviously much smaller and lighter and suitable to a lot more use cases.
Both headphones offer quite smooth full spectrum audio, where it’s a matter of personal preference if you think the sub-bass lift of the APM is appropriate (I think it is, since headphones can’t give the physical part of low bass).
Both have quite low levels of distortion.
I find the APMs much more convenient at home, no fiddling with achieving good fit to the ear canal, and they don’t stimulate ear wax production. Their noise cancellation is also better which is a boon when travelling. Plus, I really like green, even though I know they’ll look grimy before too long. 😀
Sonically I have a preference for the APMs but both are clearly very competent. The practical differences between them are vastly greater than the tonal ones particularly in this day and age of high quality digital EQ.
IMHO, and on my head of course.
Having both, I don’t need to choose. If I could just have one (which is bizarre, I have at least ten pairs of quality headphones. Which admittedly is even weirder) I would let my main use cases dictate the choice. Where I live the APPs are a third the price, so if you can afford the APMs, adding the APPs for portability is a good option. If the price of the APMs is painful, then staying with the APPs is an excellent option.
At the end of the day other people cannot say how these will compare sonically on (in for APPs) your head. You have to try yourself.