These are two of the most popular and highly acclaimed headphones within the $100-150 price range which offer excellent sound quality and are a much superior alternative to high-end celebrity-endorsed headphones.

  ATHM50x

HD 598

Tone Mostly neutral, but slightly focused more on high and low sounds. Mostly neutral, with good lows and highs, but the mid notes are more vibrant in comparison.
Ambience Offers a very dense and  closed straight-to-the ear sound. Offers a light, airy, three-dimensional live-concert like sound.
Build Quality I’ll give this one a tie.  Both are very well done.  M50x’s headband is superior, but I like HD598’s cans better.  Both have good earpads which last for a good while, but do accumulate some wear and tear.  
Comfort Offers a relatively tight fit, which favors small heads. On my head, I can start to feel a little squished during longer listening sessions. Is friendly for people with large heads but adjustable too.  Concave ear pads are excellent (and help secure the headphones into place).
Cable Detachable.  Comes with different sizes.  Many audio cables fit it. Only had 1 super-long cable designed for audio interfaces and a 3.5mm jack dongle.
Portability The ears are foldable and it comes with a travel bag. Headphones and cable are large.  Less portable.
Noise Cancelling While music is playing, you would probably have to be near a lawnmower to hear any external noise. Apart from the audio itself playing, the headphones do very little to block noise.  However, in some situations I don’t want to be completely oblivious to people around me.
Noise Leakage Some noise leaks, but as long as the volume isn’t turned up extremely high, microphones and people nearby you won’t be picking up much noise. People nearby you will hear some of what you are listening to.  Your microphone will also pick up the background noise.

I own both of these and I love them for completely different reasons.  For my a home studio, I have roles for each of them: ATH-M50x for recording vocals and HD 598’s monitoring them.  I also like to listen to mixes on each to spot differences, and to change the sound up to prevent ear fatigue.  For music, each of them specializes in different genres.  I prefer heavier and more hardcore music on the M50x while more atmospheric and fun comes to life more on the HD 598’s.

ATH-M50x are designed to be the ideal headphones for on-the-go.  Wear them on the bus or at the gym.  Take them with you in your backpack with ease.  Use them to tune out noisy family and room mates.  I think they’re a solid consumer product competing against mainstream headphones like Beats.  HD 598’s, by comparison, are designed for listening to music at home, by yourself.

In all honesty, while the ATH-M50x sounds excellent, I much prefer the HD598’s sonically.  The open-air design makes the sound feel very natural, and when I close my eyes, I can picture myself actually being the room that the music was recorded in.  The HD598’s offer a 3D surround sound feel in a very pleasing way.  The way that the bass gently vibrates across every side of your ear rather than punching into it (like many other headphones do).  Some songs, I’ve completely changed my opinion simply because I heard all of the little details which I couldn’t hear before.  As far as the noise leakage goes, sometimes I enjoy the added sounds of trees and wind blowing.

I understand that sound preference is a subjective quality and I would agree that the M50x is a more well-rounded product in overall design, but for me, the sound quality is usually the most important thing to me.  I love my ATH-M50x when I am in the mood to listen to music on them, but I always keep coming back to my HD 598’s because it provides those extra sounds and ambience which the M50x just doesn’t provide.

ATH-M50x are recommended those who are concerned about portability and noise.  However, you know where I lean on this one.  🙂