It is old post....but it would be nice to answers everyone's question I think, people...
Difficult question, but in general: both have pro and cons. Depends on what you listen. Some songs better with IEM, others better with headphone.
I myself have: Campfire andromeda + Beyerdynamic DT880 600 ohm. FIIO Q3 DAC, amp. Flac 44.1 16/24 bit files. Electronic music with and without vocals life and studio recordings, acoustic life and studio music, my music files are very diverse.
Campfire soundstage very cramped vs Beyer in some songs. And sounds are muffled, less clear. I am not rained my ears to be like professional, but in some songs there is just no competition against headphone. In other songs it does not matter which you use. I prefer clear soud, as natural as possible, as if the person is near me. I like crystal clear sound. normal bass, good instrument seperation.
I am just a normal listener with some greenhorn requirements.
Sometimes I regret it that I bought more expensive IEM, the quality of the headphone is just nice. But big and heavy.
Prize difference: 1000 euro vs 250.... Next time I will just stick to cheaper IEM. for portability it does not matter if I have BEST sound. Many times I believed the reviews of people, but I did not agree with some reviewers....
So, unless you can try many headphones and IEM during the same day. Advice is impossible to give. Your ears are different then others, what you like to have in sound quality is also different. Also, some people have ear damage because they wentto disco a lot, wthout protection, ears busted.....shame....