I will soon be buying a pair of headphones to be used for mastering and personal listening. I have it narrowed down to these.

AKG K271 Studio $180
Etymotic ER-6 Earphones $100
Sennheiser HD280 Pro Headphones $100
Sennheiser HD580 Headphones $200
Sennheiser HD595 Headphones $230
Shure E3c Earphones $180

Apart from the sound quality, my primary concern is that they be extremely comfortable during long periods of time. Also, I don't want to pay 2x price for a 10% increase in quality, as there are other things in which that money would be better spent.

So, which do you recommend?

Why not Beyerdynamic DT250?

I f you want comfortable headphones have a look at the Beyerdynamic DT100 headphones.
I use them all the time at work (I work in a Community radio station) & have used them for hours at a time.
Any thing is they a bit pricey.

Consider if you want open or closed/sealed. Open headphones tend to be a bit more comfortable for long-term use.

You can't really go wrong with the Sennheisers . Maybe consider the HD-25s - I dunno what the collective view on them is around here, but they are quite comfortable and have a lot of isolation.

Don't buy the ER6s unless you've tried proper in-ear earplugs and are comfortable with them. They just don't work for some people (I use the Etymotic ER-20 earplugs a lot and I find them slightly uncomfortable - the ER-6s fit the same way).

And yeah, I second the Beyer recommendation - there's a reason they're the standard in TV - they're really quite rugged and comfortable.

the erg's if u can handle in-ears. the frequency response graph speaks for itself:
http://www.headphone.com/graphs/showGraph.php?graphID=477
but for closed go with the HD595's