Audio-Technica ATH-M50S or Shure SRH840

M50, i prefer punchy bass and its what the M50 are capable of

Both are good. There's a ton of threads on this.

Get the m50 if you want more bass and a headphone that's slightly less detailed, but has a more comfortable fit. I don't find it all that neutral, but it's still pretty good. This would actually be better for longer listening sessions over the SRH-840. Soundstage is pretty poor even for a closed headphone. Vocals are so-so, but I wish they were a bit more forward.

Get the SRH-840 if you want a more neutral headphone with a lot of detail, but nothing excessive. The highs can be fatiguing for me at about the 2 hour mark. These are huge and don't fit that well on my head, so I had to sell them and downgrade the overall sound quality a tad and get something I could actually wear comfortably (M50). Possibly the best headphone in the world for  "Imogen Heap".

I'd only suggest getting the SRH-840s if you can find them under $150. They are not worth $200 to me. Even for $150, that's a bit high. These would be a good deal for $130.

BTW the bass on SRH-840 is good enough for me, just check the charts and you can see that it definitely has a lot of bass, but some people may still find it disappointing.

If you want even more bass (then even the M50) get the Shure SRH-750DJ. I hated these headphones though, but a lot of people prefer them over the 840s.

None of these had my perfect sound signature, but both are very good and you can't go wrong with either of them.

I have had the Shure SRH 840s for a few months and am very pleased with them.

VERY comfortable for my big head, build quality is excellent.

As for the sound, well tastes differ, but I do think they present a pretty accurate version of what the recording engineer was trying to achieve, and that's the bottom line for me.

Isolation from the outside world is good, not perfect, but I can't hear the TV playing in the same room, which is what I want.

Oh, and I paid €129, online here in France, which seems pretty reasonable.

The SRH840s have plenty of bass - at least they're good enough for me. It's just not the boomy, headrattling bass that mimics being in a club or disco.

If bass is a very high priority on your list, then get the M50s, otherwise, I'd recommend the Shures. As for build quality, just comparing the two phones at a store, I found the 840 to be much sturdier and that it uses higher quality plastic than the M50s. I didn't like the feel of the M50s and they felt more cheaply made to me. That was my first impression when I walked into the store knowing nothing about their sound signatures.

I bought the M50 instead of the 840 a month ago after trying them out in a store.IMO I think the M50 has much better building quality then the 840. I just didnt like the cable sticking out next to the ear cup, I almost certain you can early break them if you take them on the go (keep that in mind, Shure has bad reputation in cable quality). 

I personally prefer the M50 in sound wise too although I think they are very close match, M50 has far better base, deeper, powerful but not over. 840 its better in clarity. It really depends what you like in sound.  

You should find this article useful:

http://www.headfonia.com/closed-cans-shootout-m-50-esw-9-t50p-hd25-1-beats-studio-srh-840-srh-750dj-k181dj-and-dj1pro/

It's a comparison between quite a few closed headphones in a similar price range, including the M50 and SRH840.