BigbearI have both. I think the HE4XX are a little brighter, and slightly more revealing. I think you can't go wrong with either, just based on preferences. If I had to choose only one, I'd go with the HE4XX - but that's only because I prefer the brightness (my headphones at work are Beyer DT880, if that tells you anything about me).
BigbearI have the 6xx on the way as well and listen to Christian worship as well as r&b so I am curious as well if it is with it to get both for a different experience
FrostyPI haven't received mine yet but if you read the comments of those who have, no one has reported any issues with the highs. I also haven't heard of the "signature piercing highs" on the Hifiman 400i's. However if you listen with Beats by Dre's for too long, you may have adverse reactions if you hear anything above 250 Hz.
BigbearI would go with the 6xx for female vocals. They're much better for mids and classical instruments. Planars are a little too clear for those. It does get to a level of fatigue where you do get tired. The amp also makes a difference. If you have a sterile amp and planars, you'll get bored and tired of the sound really quick.