I own both. I find myself using the V6/7506 mainly in two situations: when I need isolation, and when I need to hear every last detail and fatigue isn't a problem. Unfortunately for the V6/7506, much of the "isolation" time has been taken over by Etys
So my V6 don't get as much amp time as they used to.
Don't get me wrong, the V6/7506 are great headphones, and for someone looking for a good sub-$100 headphone, they're one of those "can't go wrong" choices. But given my collection of cans, for every good thing the V6 does, I have another headphone that does it better.
As for the poll, I prefer the KSC-35 most of the time simply because they are more enjoyable to listen to. They are much more comfortable and much lighter -- I often forget I have them on. In addition, whereas the V6/7506 can sometimes be analytical, the KSC-35 are just musical. The bass isn't as extended (although better than many other headphones), and the detail isn't quite as sharp, but they always sound good.
In fact, that's one area where the KSC-35 beat the V6/7506. There are times where the Sony just don't sound that good with a particular piece. Maybe they bring out too much of the problems in a recording, or maybe the recording just brings out the faults of the V6. But I don't think I've ever listened to something that didn't work well with the KSC-35. Maybe a few pieces with overdone midrange would sound better on the V6.
And here's something that's sure to be controversial
Out of my Max, I almost always prefer the KSC-35
A few people have mentioned how, unlike many cheap headphones, the KSC-35 really do sound better with better sources and power. Well, let me tell you... when you run the KSC-35 off a Max 2001, they suddenly get amazingly better detail, and significantly tighter and deeper bass. Just for kicks, I've been listening to them all night on my Max, and, frankly, it's amazing how good they sound. I've been listening to "Time Out" by the Dave Brubeck Quartet and loving it. The biggest difference is in the midrange: the KSC-35 are a bit warmer, while the V6 are a bit recessed. So they end up having very different sounds; I prefer the KSC-35's take.
(I think this wins the award for biggest amp-to-headphone cost ratio -- about 60:1
)