The x001 has a back button.
I prefer the set of 6 buttons on the left rather than the one that the x100 has...though that is a bit of a cleaner look.
The volume control knob on the x001 is a bit of a pain in the *** but not worth getting upset over.
Honestly I bought the x001 for its non-cd use, I use a usb drive, and ipod use without need for additional hardware and the cool ability to show a picture in the display. After nearly a year of use I seldom pay attention to the display lol. Otherwise I like the x001 quite a bit.
If I had a wish I would prefer a head unit that did not have a built in amp and use the room to improve sq and tunability hardware.