I had an opportunity to compare my X3 with an AK100 a few days ago: I listened to the exact same FLAC files on both devices (put my 32gb card into the AK100, and copies of the tracks were present on the X3s onboard memory), and listened on both my old favorite IEMs, the SE530s, and demoed the shop's best $2000 cans as well. Truthfully, I could not hear a significant difference between the two devices (well, any difference, to be honest).
So, my conclusion: the real difference was that for another five hundred dollars, what A&K offered me was a better UI and little more. (I'm sure some of you will disagree, and that's ok too; those with a keener sense of hearing may discern the SQ differences between the two - but I couldn't. You could say that the AK100 is a more "polished" device, design-wise, and if that is important to you, then it's a valid point) Is the improvement of UI worth five hundred dollars to me? No. I was happy to have had the experience of comparing the two, and found the touch screen interface far more user-friendly, but I came away from it without buying an A&K device. I would be remiss if I didn't also mention that the X3 has the bonus feature of a line out, something the A&K devices do not. That point also influenced my decision to not buy an AK100.
Will I graduate to an X5 this spring? That depends how comfortable I get with the X3 in the next few months. I've found the device kludgy and a trial to actually use; the UI is a compromise, mainly because FiiO chose to forgo the touch screen interface, relying instead on an older style of button navigation that many manufacturers eschew in this age. I realize the button interface was chosen to keep the cost down, but it isn't doing the device any favors, IMO. (If FiiO could give us a player that included a touch screen navigation interface, and do so for under $500 they would have my undivided attention, for certain.) I'm likely to wait till I read reviews of the X5 before I decide what I will do about choosing an upgrade. For now, I will work at training my brain to use the X3s awkward navigation as best I can, and see how I fare over an extended period.
Thing is that the AK100 can go as high as 162GB of space while the X3 can only do 64gb max. And when SD cards higher than 64 come out it will do much more than that. Space is a feature that carries a premium.