I know Matt loves his 400s. But here it is anyway..
If I had to but only one - I would buy the MD. Why? Better tonal balance across the spectrum. Although the 400s are a very fun headphone especially for rap, where the HE-4 is not and better at vocals and classical IMO. I just can't take the constant bass jabs and then the out of know where ice pick treble to the skull the 400s dish out. While with the MD my entire listening experience is on the same level with just a slight tilt downwards of the bass. Which make them fun enough for me. Plus IMO the MD goes with more music geners when compared to the 400s.
I'll have a hard time picking the MD over the 500s though and in the end probably won't, and not a chance against the HE-6. Now the MD and the 650s would be a dog fight.
Back to the 400s. They have a really big following and rightfully so but for me that amount of bass all the time is just a bit much for my taste. I'm a more neutral with a slight favor for bass kinda guy. That's why I would pick the 650s over the 600s any day. This is the same reason I'm not a big fan of the LCD-2.2 and love the LCD-3s. Also the same reason I'll pick the MD over the Paradox any day of the week. However, it will be cool to use the Paradox as a listening tool because of it flat like FR. Just not fun enough for me. I hope this all makes sense.
I really want to take a listen to the AKG K702 65ths..