Comfort: I wear glasses and have a relatively big head so this is important to me.
ATH M50 (the original-not X): heavier than I feel they need to be and the clamp force is unreasonably tight. Can only wear for ~1hr.
5.5/10
MDR-1A: They're light and fit wonderfully. Feel slightly looser than the MDR-100aap. Can wear for hours.
9/10
MDR-100aap: These are also light and fit great. They clamp just a bit tighter than the MDR-1A. The top headband also isn't quite as comfortable as the MDR-1A. Can wear for hours.
8/10
Durability: This isn't at all objective as the only way to tell is to intentionally abuse each pair. Sometimes a durable-feeling pair of headphones have a weak joint or a light pair of phones are more durable than they feel.
ATH M50: Heavy, flexible, and built like a tank.
9/10
MDR-1A: Feels premium but doesn't feel like it can take abuse like the ATH M50.
7.5/10
MDR-100aap: Same as the 1A but the lack of substantial swivel is odd.
7/10
Aesthetics: Very subjective. However, I think it would be hard for anyone to argue the ATH-M50s are better looking than either of the Sonys. I would never go out wearing the Audio Technicas.
ATH M50: 6/10
MDR-1A: 9.5/10 (these simply look like very expensive headphones)
MDR-100aap: 8.5/10 (these don't look particularly premium)
Sound: This is also outlandishly subjective. Please read my headphone experience below to gauge how applicable this is to you. I think my thoughts will be relatively consistent with most of the reviews out there.
ATH-M50: These initially sound fun and have better separation in the mids. But the harsh high-end and the slightly sloppy bass made them fatiguing for long listening sessions.
7/10
MDR-1A: These sound very similar to the MDR-100aap but have a more aggressive low-end. Overly aggressive in my opinion.
8/10
MDR-100aap: These basically sound like the MDR-1A if the bass were more subtle. These are definitely NOT bass-deficient headphones by any stretch of the imagination but it's more tight. The word i'd use to describe these headphones is balanced. These aren't for bass-heads. These aren't for folks who want very well-defined and pronounced highs at the expense of everything else. And the mids are solid but nothing spectacular. It's the fact that it's all so well-balanced that make these great for so many people. Initially, they seem to lack the life that the ATH M50s possess but this also means you can use these for longer stretches- they aren't remotely as fatiguing. It didn't take much time for these to grow on me. They are absolutely worth the money. These headphones don't inspire confidence at first but grow on you very quickly. If they were the same price, these headphones make the ATH M50x nearly irrelevant in my opinion.
9/10
My experience with headphones: Sennheiser HD 515, Ultrasone Proline 650, Audio Technica ATH M50, Sony MDR 1A (briefly for comparison). I've also tried several other headphones in the past. I found Sennheisers to often be too dull on the high-end- I understand what folks mean when they refer to the Sennheiser veil. I got bored with most of them. The Ultrasones were heavy, uncomfortable, and sacrificed the mid-range to get a better perceived soundstage. Sold those.The ATH M50s were my daily drivers for two years but I always went back to my near-field Presonus studio monitors because the headphones were too aggressive and they fatigued my ears. What I will say is they aren't as misty-sounding in the mid-range and have better separation than the Sony headphones. However, the highs sound more shrill and the bass is more sloppy than the MDR-100aap. The Sony MDR-1A were great but the base was a bit excessive for me and lacked a bit of clarity. I wanted something more subdued. I've also tried AKG & Grado headphones and have always hated their sound signature- the high end was overly pronounced and the bass was a bit anemic. I just don't like them. I spend ~3/4 of my time with my Presonus Ceres 3.5BT nearfield studio monitors.