which is better for my sennheiser hd 598's
also what cable should i get
As the Fiio E17 comes with a nice built in DAC, it should improve audio quality better then the Fiio E11, for the HD598s.
What will you be plugging the E17 into?
Get all three cables for connection(s), USB, optical & coaxial.
The Xonar DX (CS4398 DAC chip) and the Fiio E17 (WM8740 DAC chip) come with good DAC chips.
Personally I would just get the Fiio E11 ($50) with a few extra batteries (BL-5B) and a simple AC battery recharger.
Plug the E11 into the Xonar DX's line-out/headphone jack (green).
Unless you have other reasons for getting the Fiio E17 ($140)