Eh, I can't believe nobody's said this yet--HD 600 for portable use???
They'll leak sound in and out. Terrible idea for portable use, even if you don't need that much isolation. Also, there's usually concerns about amping the HD 600; in general the amps that people would agree upon for this headphone don't come cheap.
Why these 2 headphones in particular? Why not something like a Senn PX-200?