R70x was a lot lighter and the headband was more comfortable. For HD650, I had to wear them for quite a few days before they became comfortable, but the earcups are much larger and completely covers my ears. Soundwise for me R70x was much brighter but that’s just because of the sealing issue

Just wanted to update for potential future people who find this post, what the others said is indeed correct. I compared the headphones to HD650 directly, and the 650s had much more bass. However, the frequency response graphs show that the R70x are supposed to have stronger bass. The overly small earpads are the only reason I can see.

(In fact, the R70x feels like it’s built for people with smaller heads in general. I imagine it must be insanely comfortable for them, due to the innovative headband.)