For those of you who are interested I've had the chance to carefully listen to both headphones over the last two weeks and have drawn some conclusions. Both are coming through computer RCA onboard (24bit, 48k, flac). Both are in the $200 range.
 
Where the Senn HD380 Pros win:
- Sub Bass. I was quite supprised to hear how well these phones articulated the sub bass compared to the M50s. They had a bit of punch, though it was not overpowering.
- Comfort. The HD380, as far as I can see, are not dissimilar in ergonomic design from the HD 600/650. A big and relatively deep earcup ensures that the ear doesn't even touch the sides - very roomy. These are also VERY light compared to the M50s. I can wear these for much longer periods of time than the M50s. The M50s are much heavier, clamp harder and squeeze the ears more than the HD380s.
- Instrument separation. These are, to my ears, brighter and more forward than the M50s. I can hear the guitar stand out from the mess of drums and bass. I can hear the single flute in amongst a range of violins. You get the point.
 
Where the Audio Technica M50s win:
- Bass. Where the HD380's lack all bass except sub bass (though not unappealingly so) the M50s have that in plenty - whether or not it is tight is a different question. To me it gets flabby in some frequencies around the lower mid. But, there is no competition about amount of bass and impact. The M50's also respond better to a graphic equalizer bass boost - can be quite powerful.
- Build quality. The HD380's headband looks like a thin strip of cheap plastic with a little (but comfortable) cheap padding stuck to it. The same cheap looking padding surrounds the earcups (again, its very comfortable regardless). Looking inside the earcups the veil is extremely thin and cheap looking. Doesn't seem to effect sound for the worse though.The M50's look tough, and I'm sure they would last many potential mishaps the HD380s would suffer from such as bending the headband.
- Looks. In line with this the M50 look a lot more expensive - they loo like they cost twice as much as the HD380s to me. The HD380s look good on, but not as good as the M50s. I wouldn't like to wonder around with these phones on my head given the sheer size of the earcups.
 
PS: Not much soundstage on either.
 
Overall conclusion:
 
Even though I own the M50s and am just borrowing the HD380s (and am obviously predisposed to defend my own purchase) I think I would actually prefer the Senns if I had to choose one. I thought I was a bass head, and I do enjoy it sometimes for some styles of music (the HD380s does not sound too great with dubstep and bass-reliant genres), but I am in love with the comfort and musicality of the HD380s. I could listen to them a lot longer than the M50s. I think the enhanced bass muddies the rest of the composition with the M50s and I didn't really realize this untill now. Don't get me wrong, for the price the M50s are great and I love them. But I think I would rather have the HD380s for about 20$ more I think. Apparently these phones also benefit more from a decent amp than the M50s so it might actually be a lot better than what I have heard so far. One is not better than the other - as with most phones, one must choose in line with ones musical and audio preferences.
 
Hope this was insightful for some.

Great review. Never tried the ATH 50's but I can vouch for the HD 380 as I own a pair. Bought them at the recommendation of a friend. I was not happy with them initially but grew to like them over time, now they are my favorite even beating the HD598. They do benefit from a headphone amp. Poor quality DAC's on PC's and audio devices will definitely be noticeable with the HD 380. Wearing comfort is also good in addition to the passive noise isolation. I can use this in the train without much issue of noise bleeding in or out. My Sony Xperia Z smart phone can power then well enough but an additional headphone amp will be helpful. I also noticed that the headband seemed to be not so durable. In fact this is why I don't use them as much as I feel that the stresses of travel might break them.