Heya,
They're polar opposites of each other.
HD650 - Dark, Sleepy, Smooth, Bassy
SRH940 - Bright, Massive Mids, Detailed, Clear, Bass shy
I have both. I would suggest the SRH940 to someone who wants a headphone for acoustic and female vocals pretty much exclusively as that's where I found them to shine the most. If you want a headphone for all genres, the SRH940 is likely not it, I didn't think it had enough low end to support electronic and some various musics that require a lot of bass reverb. It's a detail monster with a lot of mids and pretty bright, but not sibilant. Again, great for acoustic and female vocals and some jazz. Not the best for rock and electronic and other stuff. The HD650 on the other hand is good for most everything, it's a very relaxed smooth headphone to listen to, which some people love and some people do not like. It's pretty bassy and it takes a lot of amplification to wake it up, but once you get there, it's a great headphone, it will work well with everything. But again, it's not a detail monster nor something I would call overly clear and detailed for analytical listening, it's a musical headphone, fun, very smooth. Between the two, I would probably suggest the HD650 to someone who didn't know what they really wanted because it's more likely to cover more genre ground than the SRH940. If you wanted listen to acoustic and female vocals however, I'd put the SRH940 over the HD650 with authority. So again, they're not really comparable. That's why you didn't find anything on search.
Very best,