I've owned both (V6 instead of 7506) and I preferred the Sony.
It really depends what type of music you like though, not sure what I was thinking buying the SR80i cause I hardly listen to rock..
I absolutely wouldn't discount the Grados just because of build quality or comfort! They're very basic in their design, and while they aren't the most comfortable in the shape they are out of the box.. The headband can be bent in any way you want, once it's bent in a way where it just lightly rests on your ears it's quite comfy. I could wear it for longer than my V6, just due to the light clamping force and the fact that my ears stayed ventilated.
They're very different headphones in terms of design and sound. The Sonys have very sharp highs (some people can't stand the sound) and nice bass, but recessed mids. The Grados aren't very bassy at all, very pronounced mids, and average highs. The Sonys are closed cans, and they fold up.. The Grados can lay flat, but they're completely open. The V6 have a decent soundstage for a closed can, the SR80is have absolutely no soundstage. Also, the Sonys got better with an amp, the Grados didn't.
What made you pick the 7506 over the V6? They are nearly identical, but the 7506 costs more.