I have the Sundara and HE5xx. I like them both but the Sundara is superior in Sound Quality. They are both very comfortable to wear for hours. The strap design works well for me. The Sundara has a slightly higher clamping force on my head but both are nice and light weight.
I recently purchased an HE400SE. However, I was kind of dissapointed in the SQ. I expected it to be lacking compared to the Sundara but figured it would be equal and maybe a bit better than the 5xx. I also did not expect them to be as uncomfortable as they were. The headband design just did not work with the shape and size of my head resulting in a hotspot that actually became painful after 30 mins or so. I've read that this goes away after some time and I expect that this would be the case - either the band softens up or my head would get used to them or both.
In the end though, since the SQ was not distinctly better than the 5xx (really less IMO), they went back. Over a few days and a number of hours of listening, the 400se just didn't work for me. If I didn't already have the 5xx I might have kept them longer to try and bond with them a bit more but for me, the 5xx was slightly better SQ and vastly better for comfort.
So for more focused critical listening on my main system, the Sundara is the phone. For more casual listening on one of my other systems and away from home, the 5xx is first choice.
Edit: I actually now have 2 HE5xx plus the Sundara plus some other Senns and Mezes. The price "Drop" on the 5xx is pretty compelling - That is a lot of headphone for $160. I paid a bit more for both of my sets even with the points credits...
