If you ask this anywhere but this board, you'd probably see the inverse of the above poll results (at least as of my posting it's over 70% in favor of headphones).
What a self-selected group of audio obsessed sound geeks will find preferable in comparison to the rest of the human population is unlikely to be the same. I don't think there's any question that at the same price points you can get better sound quality out of full size headphones. Grado SR60 versus Ultimate Ears metro.fi 2, Sennheiser 595 versus Sleek SA6, 'nuff said. However, no matter how different and how big of a sound you can get out of headphones relative to IEMs at about the same cost, the truth is that it is not just about sound quality for most people.
A lot of the posters on this board place sound quality as the number one criteria. Do the headphones need a special $3,000 amp that only powers two other models of headphones in the world?
Only a matter of saving the money to get that sound. Are the headphones too large to do little more than sit nearly perfectly still in a chair?
All the better to enjoy your music with. Does this headphone have a sound signature that only fits live jazz and sounds fantastic but otherwise sounds pretty weird with most everything else?
Sweet, lets fire up some jazz, there's nothing wrong with its sound signature. That is not how most people's mind works.
I don't believe IEMs have an
unfair advantage, but they certainly possess advantages that eliminate the question of differential sound quality for me and, judging by sales, most people. While it's not going to be a popular opinion, it's perfectly correct to look for the best sound quality you can get that works in all environments, sitting down or on the go, amped or unamped in lieu of niche use cans. In more than four years of being obsessed with music and ways to play that music, the most expensive full cans I bothered to purchase were ATH-A900, and I sold those to buy IEMs
There are times I'll grab headphones for the big, airy nature of their sound, but the bottom line is that I'd much rather be listening to IEMs (or even good earbuds) over full cans most of the time. Portability, comfort (my ears like the air), not having to worry about bothering others with leakage, clarity that doesn't take an amp and a quiet environment, etc., soundly trump any potential sound quality gains I'd get by using full cans.