I haven't listened to the 840's in detail, but if you're considering the 940's, I find that they do isolate pretty well. However, on a noisy commute (via on the bus), due to the nature of the headphone itself, the bass is almost non-existent unless you turn up your volume at medium levels. I'm not sure if you get THAT much ambient noise where you are, but I thought I would mention that. I personally don't mind the lack of bass since my on-the-go music doesn't place an emphasis on the bass, but more on the mids and highs. Also, because the 940's place an emphasis on the mids and highs, they can be fatiguing to some.
For a review of them, the link is in my
signature.
I think for a noisy environment, the cheaper 840's will probably be a better choice since they do have a mid-bass bump, meaning you would probably be able to hear some bass much better than the 940's. For quiet, at-home listening, I would recommend the 940's if you want a detailed headphone.