This thread has some thoughts on the 650's vs HE-400.
http://www.head-fi.org/t/636629/sennheiesr-hd-650-vs-hifiman-he-400
I was personally in the same predicament last year and ended up going with both of them around Thanksgiving. 6 months later...the 650's get infinitely more play time than the HE-400. The HiFiMan's are great headphones...don't get me wrong, I love their unique sound. However I prefer the comfort and sound signature of the 650's more. The 650's in my opinion are a more laid back headphone, whereas the HE400's would probably be considered more 'fun' or in your face.
I prefer a lot of Jazz, female vocals, acoustic, classic rock, etc.. music. The HE400 does fine with these genre's, however I think the 650's excel at these genre's. Both headphones are different enough that I think you can make a good case for owning both, however if you're only looking to get one pair I think I go with the 650's. I think if your into EDM type music more, then the HE400's may be the way to go. Like I said, both headphones are fantastic...but if I had to pick a winner I easily pick the 650's. Pairing the 650's with the proper amplification and I think they are pretty hard to beat. Obviously if you have an opportunity to demo both of them and chose that would be preferable...if not my vote goes to the 650's.