The E7 vs E11 most likely won't be much of a difference, as the E7 won't override your iPod's DAC like it would with a computer source. I've heard the E17 is a huge improvement though; you'll still be using your iPod's onboard DAC but from what I understand the E17 really improves the sound tremendously. Not 100% on that.
I liked the E7 due to it's 2 output's which would help when comparing headphones as I keep losing my splitters and will also be good when showing someone a song and letting them plug in their earphones. However, I read a few reviews saying it sounds quite thin compared to the E11 which gives a much warmer sound (I like warmer sounds) which is why I'd probably got for the E11 but I've seen the E17 and I would like the ability to change the EQ so I'm tempted to go that route.