I personally always go with open-back phones, like my current fav, HD600s. Here's a comparison with DT770s pros:
The Sennheiser HD 600 and the Beyerdynamic DT 770 PRO can both be considered as audiophile options but have very different strengths. The Sennheiser have an open-back design. Their passive soundstage is much more open and spacious-seeming, and they have more consistent bass and treble delivery...
www.rtings.com
As you can see, except for the lack of bass, HD600 has better scores in Soundstage and Trebble.
As far as looking at Headphones impedance, I found it, in general, unrelated to sound quality, except in cases where your headphone amp would struggle to power them like most amps won't able to power 600ohm phones to any normal levels.