Theres a distinct difference between the er6i and the er6...The er6 is supposed to sound more like the er4s, whereas the er6i is basically the er6 but with an 8db boost in the bass-midrange frequencies. The er4s is etymotic's most accurate iem, it's also therefore the brightest, most detailed. The er4p is a more sensitive, lower ohm version of the er4s with less accuracy.
This is all theoretically speaking, according to etymotic. In actual usage, people prefer the er4p and er6i for portable use and the er6/er4s if you have very high quality sources and prefer the most detailed sound.
The er4p is a compromise between all of their models, it has accuracy closer to the er6/er4s and higher sensitivity than the er6i.
The er6i's come off as warmer, more sensitive and more suitable for the budget mass market ipod users.
I've had the er6 and have to say that it is very low in sensitivity, you'd have to drive your ipod, in my case an iaudio u3 pretty hard to get loud music, but then again i do like to listen loudly, not unbearably loud, but loud occasionally. Personally, I didn't like the er6, way too bright and unsensitive for my ampless taste. the brightness pierced my ears when i turned it up louder than normal.
I stuck with my Westone UM1s with some slight eq.