From my short impressions, the HD238 has a slightly larger soundstage, which is kind of inherent of it's open-backed nature. I feel the PX200-II feels about as "open" just not quite as wide of a soundstage. Positioning seems better on the PX, bass was tighter if not as ample, and I heard more clarity(this last one is unfair to judge as the HD238s were auditioned in a quiet but still somewhat noise best buy). I would prefer the PX200-II from my short impressions. Their added portability, volume control, and seeming durability is another plus.
Again, take my impressions with a grain of salt, I only had 15-20 mins with the HD238 through my cellphone at a best buy store.