The ER6 need an amp to sound really good and some EQ (for me) to sound beautiful. They tend to be thin in the bass and edgy in the highs. The ER6i take a good step in dealing with the weaknesses of the ER6. The ER6i have a fuller bass and are less pronounced in the highs. If you're looking for something to work well with an ipod, it's worth it to go with the ER6i. And yes, the ER6 cable is quite microphonic, making very noticeable noises whenever it rubs against anything with texture, like your shirt. Again, the ER6i takes a good step in dealing with this weakness.
I use the ER6 with an amp (that is positively synergistic with them) AND use EQ with them in order to make them sound what I would call beautiful. Plus, I am always seated at a desk when I use them, so the microphonic cable problem is rarely noticed by me. The ER6 was Ety's first attempt to come out with a "budget" version of their ER4x's. In designing the ER6i, Ety seems to have taken the complaints they've received about the ER6 into account - with steps taken to make improvements in the weak areas.