It depends. Some 'stats will lack impact, and some dynamics will lack details, and headphones of either type can have all sorts of frequency response irregularities that will color them in all sorts of different ways. But as you get closer to the top, 'stats will have impact, and dynamics will have detail, so what's better will really depend on how much you like the sound signature of the headphone in question more than anything else.
Still, for pure technical aspects of the presentation, at the top end 'stats will win out - they'll have all the impact but are faster and more detailed, and simply tend to disappear more and let the music come through without seeming to impart much of the system to it. The margin is small, but it bothers some people enough to be turned off by dynamics altogether.
I'd say go by the individual headphone, listen to it in a proper rig, and forget for the time being what kind of driver principle it uses. But that's a loaded statement since "proper rig" can be very tricky, especially with a headphone like the O2.
Speaking of which, for me the O2 (Mk1) is as accurate as it gets, but as with any truly transparent transducer you can impart to it any kind of coloration you want by matching associated components. In my rig I have it subtly colored in a way that I like, not far off neutral but definitely on the lush side and with a bit of tube glow thrown in for good measure. But that's because my amp is hardly a straight wire with gain.
