Depends on what types of music you are going to be listening to most of the time. I owned the 840's for a year and I currently own the 940's. I listen to a lot of Jazz/vocalists/indie and some classical on these headphones and I by far prefer the 940's for these genres and think they are worth the difference to me, but if you listen to more rock, metal, hip hop, rap, etc, your opinion might differ if you listened to both pair of headphones.
The 840's are fairly neutral, good all arounder headphones. They have slight mid-bass emphasis, but work well for a range of genres. The mids are the highlight of these headphones, bass is good, but sub-bass impact may not be enough for bassheads. To me the 840's had a decent amount of detail and were great headphones for the price (although they going for a lot cheaper $130 when I bought them and since retail prices have gone back up). They never had the wow factor for me, but they didn't do much wrong.
The 940's to my ears sound much more detailed and have a lot stronger treble presence without sounding harsh and fatiguing. Mids are great on the 940's, a bit more detailed and smother than the 840's to my ears and the soundstage sounds a little bit bigger on the 940's to me. Bass is very accurate and tight, but not as present as the 840's and has less impact. The bass on the 940's is great for me, but again I am listening to a lot of jazz/vocals. I don't enjoy the 940's as much with Hip hop, hard rock, dubstep, etc. So if you value bass impact, these headphones probably wouldn't be a good fit for you.
Well, I generally listen to classical, choirs and quartets. I'm not a basshead, but enjoy having a rich bass sound. (One that does not give me a headache after 4 hours of listening). But most of the headphone's use would be for gaming. I'm also a musician and would use the headphone on my digital piano, once in a while. So I guess I need a good sound range, not to miss a good bass or feel annoyed by some distorted treble.
I bought a sennheiser HD558 and loved the sound, but got really annoyed by the open back design, with everyone listening to what was going on inside it. That's why I want to buy another pair. I was looking for headphones around sennheiser, shure and Bose, and these two shure's called my attention. Both because they are closed backs, with a brand I already worked with, and also are at a pretty accessible price.