The Cowon Plenue D (PD) is definitely not as good as the M5, in reference to details, sound stage, imaging, and analog sound. The PD sounds more digital, (too many digital artifacts), however, one thing the PD does that the M5 can't is mind blowing amounts of Bass. If you're a bass head you need to look no further. It's nice to just thump the bass to extreme levels and still have enough clarity to claim it sounds better than smartphones. Of course you'll miss out on the details, but if you're listening purely for the bass, who cares about the details at that point.. Lol.. The bass on the M5 is accurate and can not do the extreme bass levels the PD can do, but of course it's not trying to be a bass head Dap, it's meant for accuracy, and that is what it offers in spades over the PD.
So in the end, it is up to you whether you want a lively sound or accurate sound. Don't get me wrong, the M5 has plenty of bass, it just won't give you that deep sub-bass pound like you can get from the PD. But I haven't found another player that can compete with the PD in that aspect.