I have both, and usually use them with Sony 7550 IEMs.
I think the biggest improvement the F series has over X is the almost non existent background hiss and the ability to play FLAC files. And obviously it has Android system, so if multimedia is important to you then the F series is no brainer.
Other than that, the X has a thicker, slightly more organic sound to my ears, but smaller soundstage. I find it sounds nicer with vocals, especially jazz.
F is more flat but sounds awesome with string instruments.
To me, the sound signature difference is more than actual sound quality improvements.
Edited by kwang411 - 11/11/12 at 12:22am