I have both. I had gotten the E3C's because of the perceived durability of the cords vs. the Etys and the looks (as shallow as that is). Having said that, I'd vote for the ER6i's -- the cords are actually more durable, despite seeming frail and thin, and they sound better (to me) with a wider variety of players. (I had come to accept the E3's with a highly EQ'able Rio Carbon and Rio Karma, but I don't like them with an iPod on a flat setting, for example.) In addition, I've found that the tiny ER6i's isolate even better for me than the Shures (both using foamies), which did a fine job of that.
As I've said elsewhere, I should have gotten into Shures starting at a higher level ... I think the ER6i's are a far better value than the E3C's. The list price for the Shures is appaling ... even at the apprx. $129 sale price I paid for them a couple of years ago, they were overpriced. The ER6i's, on the other hand, might be one of the best bang-for-the-buck values out there.
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Phones (in order of purchase):
Sony MDR71, Sony MDR51, Etymotic ER6, Panasonic RP-HJ50, Shure E3C, Koss PortaPro 2, Creative EP630, Etymotic ER6i, Sennheiser PX-100, Sennheiser HD555,
Sources (in order of purchase):
2nd Generation 10GB iPod for Windows (retired)
Creative Muvo 256MB
Creative Muvo 1GB (out on loan)
Rio Carbon 5GB
Cowon iAudio 5 1GB (out on loan)
Rio Karma 20GB (low-mileage collector’s item)
Creative Zen Nano 1GB
Samsung YP-U2 512MB
4th Generation 40GB iPod (monochrome; used as home jukebox)
2nd Generation iPod Nano 4GB
Creative Zen Stone 1GB
iRiver T60 2GB