You are right, I was thinking about the E18, which cannot be docked with the E09K, but the E17 can be docked just like the E07K. The price difference is not too big between the two, in Canada it's about $50. I decided to go with the E07K initially as I am not sure if I will make use of the optical line-in, and thought I'd save the $50. However, if I had to choose again I may go with the E17 just to have more options down the road.
But I think it depends on your needs in the end, as both the E07K and E17 uses the same DAC chip, so sound quality should be about equal.