I have HD700 and I love it. One advantage I found for HD700 than the HD6 serious is easier to pair with amp.
Although it is not difficult to find a amp matching HD6 serious, the high will sound unreal when pairing a wrong amp (like the HDVDs)
Also, I think HD700's bass is better.
The main difference between the 2 will be tonarity. HD6 serious has a unique color that will make HD700 sound dull when comparing them side by side. Other than that, the quality of sound for HD700 will be better.