IEM for me. Even though headphones sound staging is better, IEMs are so light, it doesn't feel like I'm wearing anything, which makes the illusion more realistic. Sold off my HD650 after a year not because they sounded bad. They were one of the most comfortable headphones I've tried, but still heavy.
I don't really think it's sensible to claim headphones or earphones are generally better. That's probably why no one's made a thread about it before. Each has unique purposes. Also, a good set of headphones is typically cheaper than earphones of comparable quality. At the top-end of audio, headphones dominate reference audio gear. Musicians need the small-profile and obscurity IEMs provide. A producer working primarily with string instruments might pick an electrostatic over anything else.
IMO, ear buds are the best Portable, comfortable, have less markup. Yuin PK3s are the best you can get at $40 without modding.
Personally, I would pick earphones. I listen to music a lot outside of my home and having my music with me wherever I go is a must. Sometimes I wish that I owned a pair of full sized cans for comfort reasons and ease of use while at home but I'm alright with IEMs.