Well, they're pretty different. The sound signatures and technicalities are quite a ways apart, so it depends on what you're looking for in a headphone. The M50x's tuning has a textbook, mild v-shape with boosted bass and treble, whereas the Planamic has boosted bass, a fairly present, warm-ish mid response, and a very rolled off treble. The former has a bit of an unrefined quality to the sound (I feel like the waves bounce around the inside of the cups a bit, and the treble gets a touch crispy), whereas the latter is very liquid and smooth (maybe a bit less so in the treble region to piggyback off on jaydunndiddit's comments elsewhere in this page) and can be further refined by using a SPC cable.
It's been awhile since I've heard a pair of the M50x so I can't directly compare the technical sound features with much authority, but I remember it sounding very closed-in, like a closed back would, whereas the Planamic has a bit more soundstage and overall refinement. But perhaps that's because it can be run balanced and the M50x cannot. Of course, form factor-wise totally different as well with tip rolling and cable upgrades being more plentiful for the IEM, further refining the sound to one's tastes. All things being equal, I'd say that the Planamic is more comfortable to wear as an IEM than the M50x is to wear as an over-ear though.