Hello, I have both and take turns sometimes using them. My opinion is very hard to decide because both have differences and for my opinion...
Beyerdynamics have a very good feel ,light and comfy has good bass for the drums and it has medium highs but it all seems to come together and I listen to my home stereo which is not a very loud one but my Sony does the job with a little help with my ADC equalizer. These have more isolation I think with some pretty big sound even at low levels. I listen to Metal,Rock,Classic Rock,etc. I love the case it comes with,I even bought an after market heavy duty one and the cord is nice and thick as long and straight. Overall the sound has more bass and may lack some sound in the mids but that may be me.
Hifiman HE-400's are strong bigger but are comfortable too I have to crank up the volume on these more than the T-90's. Another thing is they seem more open(airy) and helps but does have bass but the T-90's seem more lower. The mids and highs are good with some changing on my eq they can be less shrill I do like these too they don't fatigue the ears ,I'm not saying they are better than the T-90's The cable is cool but be careful they can come off but can put them back on unlike the others.
So if you have the money I would go for the Beyerdynamics for the music I listen to and the Hifimans aren't bad either they are a good choice but may need a headphone amplifier for better sounding. Seems there is always new ones out and many choices. Let me know if you ever listened to these and bought them or did you get another choice? I know this is an older post but I already wrote this. Thanks, Russ