Audio-Technica ATH-M50S or Sennheiser HD448

I have the HD448's and I love them.
Are you going to be using a portable player or an amp? My Sony E-series is lacking the power to drive the headphones to their full potential so they are quite quiet with little bass. I'm not sure about other portable devices though. When hooked up to my amp for home use they output a very decent amount of bass and can reach a high volume setting. The mids and highs are very good and clear but the bass can be a little bit muddy or boomy at times which might be a problem for you if you listen to bass heavy music. I generally listen to rock and metal with a bit of reggae and the bass for me isn't a huge problem and the great mids/highs kinda help even it out. They are also very comfortable to wear for extended periods of time.

I've never heard the Audio-Technica headphones so I have no comment on them but I can definitely recommend the HD448's. Bear in mind though that it all depends on the person so if possible I would recommend having a listen to both pairs before making your purchase although that isn't always possible of course.

If you want closed get the Sennheiser IMO. If you want more bass get the 555 but those are not closed if you care about that. Then do this
http://www.youtube.com/watch?v=PvQ8PzBrHXE