I have the M50x, I don't have the HD58X but I have the HD6XX (they're not the same, but share some similarities). I'll make a guesstimation of the sound of the HD58X according to the frequency response curves and some reviews, and compare them with the M50x:
- comfort is better on the HD58X, if you had stock earpads on the M50x. With better earcups, I'd call it a tie. The HD58X are a bit lighter and the headband feels better, but the M50x adapt better to your head and have less clamping force. Both are really comfortable once you get rid of the stock earpads of the M50x (I have the Brainwavz Sheepskin leather earpads, and they're great, way better than the pleather ones).
- build quality, both are great, but the M50x look more durable. Both seem easy to open in case you have to replace-resolder something, but maybe the HD58X are even easier to operate.
- soundstage is better on the HD58X, but there's not a huge difference. Open back headphones usually have better soundstage than closed back. Bear in mind that HD58X don't have a huge soundstage like, for instance, the AKG K701. And the M50x, being closed-back, have a reasonably good soundstage.
- bass is way better on the M50x, unless you want an anemic bass and almost no sub-bass. If you enjoy the bass kicking in, or wanna listen some sub-bass rumble, M50x are a clear winner. The HD58X have less bass than the warm HD6XX, so they must sound somewhat "neutral" compared to the more warm M50x.
- mids are better on the HD58X, M50x are clearly v-shaped (enhanced bass and treble response), so mids are a bit recessed compared to the more balanced HD58X.
- highs are so different, depending on your preference you will prefer ones or the anothers: M50x have bright, crisp, sometimes even harsh highs; HD58X are dark, mid-highs are slightly veiled, and highs are kinda recessed. If you prefer accurate, easygoing, non-fatiguing treble response, HD58X. If you prefer bright, exciting treble, M50x.
In the end, it all depends a lot on your preferences and the type of music you listen. I can tell you my HD6XX have almost no sub-bass, and some hip-hop and EDM music lacks that bass oomph that the M50x have. And since the HD58X have even less bass than the HD6XX, maybe you'll get dissapointed. On the other hand, vocals sound amazing on the HD6xx, much more warm, natural and detailed. Instrumental music is better on the HD6XX too, and they cause way less fatigue after a long listening session due to the dark sound signature.