I prefer single sided cables by a large margin. Some people on head-fi have noted quite unfortunately that some single entry headphones sit unevenly on their heads, sometimes pulling down on the side where the cable adds the extra weight, but I don't think that's happened to me enough if at all. I can see the argument for dual entry but frankly I don't find the logic compelling. That is to say, the logic is there but I have not experienced an advantage to dual entry cabling, in fact I find it less comfortable and less convenient.
However I do understand that from the standpoint of "high end" audio and all that it entails, a headphone design that includes dual entry wiring is done so that even the theoretical performance factors are covered. Someone demanding luxury or even perfection out of their headphone will want to be assured that the drivers have been matched to a certain minimum degree, the cups free of flat or parallel surfaces, and the cables of even length, because the theory supports these measures as bringing you closest to fidelity.
I actually consider myself lucky that I don't care, because if I started to think that I needed to get closer to audio nirvana I'd probably start imagining channel imbalances and stuff and claiming to hear better than everyone else just to justify having better equipment. As it is, I've spent less that the dollar-per-post standard that I've seen quoted on head-fi, and it's largely because I had to let go of the little nitpicky nuances that make this hobby so immersive. I pick up some gear, try to enjoy the music, and if I notice grain (KSC75) or frequency bleed (T50RP, < 10 hours) or some other shortcoming of an otherwise nice headphone, I just have another drink. Makes me so much happier than buying more gear to feed my gear. A little off-topic, sure, but the same goes for when my dual entry cables are annoying me. Nothing a double-vodka-over can't fix.
In any case, single entry to me is just so much more comfortable and easy to deal with. Heck, my listening rig is positioned to my far left, which keeps the cable out of my way, except for earlier today when I had on my HD 424X. Felt like I was wearing a seat belt, because I was aware of it the entire time, and that's not even a heavy cable. Forgot about the "vodka mod" at the time though.