The HD650 does have better mids and vocals, but the HE400 does a great job with thay too, and its more lovely in the bass and energy. I would always prefer the HE400 over the HD650 if I could only own one. The HE400 does many more things better than the HD650, but they are pretty comparable overall.
If its a rough tonal balance comparison, youre basically trading off some mids for more weight, energy, and imaging. The HE400 is more special than the HD650, but the HD650 does have some truly amazing vocals.