Actually the HD650 are not closed back...The band is Hifiman and the model in Sundara.
I don't think anything beats Sennheiser for quality of build in their price point, but the Sundara sound is more pleasing.
Both are considered 'reference' to compare other headphones to.
The 6xx are dynamic driver and closed back, the Sundara are planar and open back.
If you are going to travel with the headphones I'd go Sennheiser, if they are going to sit on a desk, I'd go Sundara.
The sound difference between the two is significant, but the Sundara and the 6xx are the two kings of their price point IMHO.