The SRH940 is a very nice headphone, and as far as I can tell it continually holds up surprisingly well to my HD650, but it's such a different headphone it's really really hard to compare. I'd say to the extent of my hearing the SRH940 is very close to the HD650 in overall sound quality (but the HD650 is a bit better IMO) - but each has their own strong points.
The SRH940 is amazing at sounding very detailed, partly due to it's sparkly highs and excellent mids - so in some songs with female vocals the SRH940 is better than my HD650. The problem with the SRH940 is that it is a bit bright (only very slightly), and it lacks a full bodied bass impact compared to the HD650. The SRH940 has very nice and tight bass, but it does not extend as far or have nearly the impact of the HD650. Also the SRH940 being highly analytical can tend to sound comparatively "grainy" especially with imperfectly recorded songs, while the HD650 contains all the detail but with a smoother presentation.
The HD650 has much better bass, and smoother mids, and definitely has all the detail and resolution of the SRH940 (more resolution IMO) but it's not as forward, so it's not the first thing you notice. What this means to someone who doesn't care to listen super-closely is this: The SRH940 will sound more impressive at first, but you'll get tired/bored of it quicker. The HD650 on the other hand has such great subtle detail and resolution, but it will just sound musical at first - and continually more impressive the more you listen to it.
For whatever reason, I almost always tend to go back to my HD650 than my SRH940. The only time I use the 940 is when I need isolation, want to listen to something where sparkly highs are important, or if I feel like listening analytically.
I just listened to Owl City "Galaxy" and Lights "My Boots" on my HD 650 and SRH 940. They sound excellent on both, but I ended up preferring the HD650 for Owl City. Edit: I originally said I preferred the HD650 for Lights but I'm actually starting to like the SRH940 better, I think due to the extra detail and crisp treble. Again I think it's kind of a preference thing - if you're in an analytical mood the SRH940 can be better. If you want to sit back and listen to the music the HD650 is often better.
Thanks once again for the extensive overview! For now I think I'm gonna stick with the HD650 once I get my birthday money but maybe later on down the road if I have extra money and want some closed headphones I will definitely consider the SRH940's!