I hope I will not offend anyone with this, and as always, YMMV.
I liked the Q701 very much, but at times its treble can feel a bit piercing and fatiguing. Also occasionally I actually got distracted from the music by its imaging. The bass sounds excellent after the mod though.
The HD600 is my favorite under $500 headphones, and ironically I've never owned one. It is very balanced across the spectrum with tight and deep bass, intoxicating mids and smooth highs. One indisputable fact remains, that the soundstage of the Q701 is by far more spacious, bested almost by no headphones under $1,000 I think (haven't heard the HD700). As tempting the soundstage advantage may sound, the imaging of these are a mess. Other headphones, like the the DT880, especially the HD600 images much better.
I will not go very far in terms of comfort, we all have different criteria of comfortability. But for me, the clamping force of the HD600 is too much that in the long run I think it may give me headache. After putting many hours into exercising the bumps on the Q701 headband, they became softer and cease to exist as a problem. I also like the fact that it's very light, and with single entry cable, more convenient as well. Q701 wins in this regard for me.
If you watch a lot of movies and play a lot of games with your headphone, the soundstage of the Q701 will come in handy. If used solely for the purpose of listening to music though, I think the HD600 would be the better choice.