If you want a good open can option I personally would recommend the HD558s (or the HD598s)
I have owned the M50s, the HD555s, and the HD558s. I listen to christian rock, a lot of acoustic music and solo vocal ballads, K Pop, J Pop. Not trance though. (Although I heard the HD558s are trance BEASTS)
I personally generally enjoy these cans more for some of the genres listed above. ESPECIALLY acoustic guitar. After I traded my HD555s to get M50s, I realized I made a mistake because I couldn't enjoy acoustic music as much; it sounded cold in comparison. the HD55X series have a very smooth and warm midrange that is not recessed like the M50. The midrange does matter, and I think if you listened to both for a while you might agree. Acoustic guitar is so important for me and the HD558s do it 5 times better for me, no exaggeration.
K-Pop and J-Pop sound much better on the HD558 in my opinion. These genres are generally pretty well produced and mastered, and the HD558 really lets you know it. The HD558's soundstage is phenomenal, and the good stereo separation of kpop tracks makes them sound completely immersive on the HD558s. The midrange makes synth lines sound very nice, and the same with pianos and guitars in kpop ballads, and vocals. If you tell me what artists you like I can elaborate more on this.
One thing I like better on the M50s is Christian rock. The shimmering, etched treble of the M50s generally resolves very heavy rock mixes a little better. If you like Christian metal (i have no experience here) I would get the M50s. If you like lighter mixes... Starfield, Switchfoot, David Crowder, Tomlin (shooting in the dark here) I think it's a matter of preference. Softer songs in Christian rock sound better on the HD558s. Aggressive rock sounds better on M50s. The HD558s have no problem whatsoever resolving thick sound, but the frequency balance of the M50s lets certain instruments cut through better.
Other notes: The HD558s are open cans. If you use them at home this is not a problem. Their soundstage knocks the socks off M50s in my opinion. but if you want to travel a lot with them get the M50s. Also, the HD558s have great bass with good sub-bass extension, enough that I don't miss the M50s' bass. The M50s have more, nevertheless, and with more impact. The treble of the HD558s is nice and smooth, while the M50s have a little extra sparkle.
Hope this helps. If you have questions please ask.