I am a very big fan of both set of cans, but each has a different sound signature. It really depends on what music you are enjoying with each, so I am hesitant to publish an opinion.
NightHawks: have an amazing linearity and
total lack of distortion - even when driving them to absurd volume levels. This is what I notice most when I come back to the 'Hawks after a break listening to my other 'phones.
They are maybe the most "speaker-like" 'phones I've personally experienced and for that attribute alone, I seriously love them. That is, nice sound stage with a presence that sounds like it's outside of your head; unlike the HD-650s (my all-time favorite headphone) where the soundstage is sort-of three blobs: center, left, right that sounds like it's right next to your head (because, well, it is).
What bothers me is the upper-bass / lower-mid bloom. With some music, it's painful and with other music it's magic. Some piano notes, in particular, are difficult to listen to with the 'Hawks. For example, Agnes Obel's poorly mixed "Citizen of Glass" is agony with these cans, but I've found that recording's emphasis on the lower-mid piano notes to be troublesome with many headphones.
With classic rock, however, the 'Hawks are my favorite cans. I've been listening to them today with the new HD Tracks release of the Best of the Grateful Dead and it's just musical and fun.
Sony
MDR-Z7: the infamous "fart cannons"; that description isn't entirely wrong with some music, but with Electronica, EDM, Trip-Hop, it's magic - really. I find the bass really clean with music like Carl Finlow - best cans I have for that artist, which has punishing lower bass.
What I like the Z7s is the separation of instruments; so even when the bass seems excessive, somehow, the mids and high aren't masked or muddled. With some classical and some jazz, the Z7s are incredible, from a detail and staging perspective. There is some warmth missing, to be sure, but that's because I am used to my HD-650s with an OTL amp - where the mids are epic.
Don't know if any of that helps, as you haven't mentioned what music you like, but there it is...