i have owned two pairs of Grado SR125's, and i still own two pairs of the sony mdr-v600's (cheaper version of the mdrv6).
sound - i like the Grado's better for sound, to me there's a lot more detail and they're very neutral. the mdrv600's are a bit boomy and woofy.
wear - grado's are light on the ear and head and so pleasant for extended listening. sony's are less comfortable and make your ears sweat. on top of that the well known sony flaking occurs after about 18 months where you suddenly realize you have peices of black plastic stuck to your skin.
spill - the grado's let in and bleed out a lot of sound. if somebody is in the room watching tv i can't use them. conversely if i listen to music everybody else hears them. sony's are closed back and don't have this problem
reliability - here is my real bugbear. i paid $150 for the Grado's and i have gotten through two pairs in 6 years. the problem is that the earpieces can rotate 360 degrees. over time the constant turning and unturning weakens and breaks the outer cable sheath near the Y connector (where the left and right combine into one cable), this exposes the inner cable to twisting and they finally break (both pairs i have had suffered this breakdown). both sony's i've had for 10+ years with no problems. now i use headphones every day to listen to music or watch tv after my kids have gone to bed so they get heavy usage. i've never taken the Grado's more than 10 feet from my stereo system, and yet, they've broken (twice).
so in conclusion, i love the sound of the Grado's but i hate the reliability. the sony's i don't like the sound as much, but i love the reliability. i would never take Grados out of the house.