I owned a RWAK100 (same idea as, and perhaps slightly better than Mk II) before the X5 and compared them side-by-side before switching. The AK100s have a slightly blacker background than the X5, but the X5 sounds superior in every other way to my ears. The soundstage on the AK100 is slightly skewed and the overall tonality is a little further from neutral (which some may prefer).
Also, the line out from the AK100 is via the headphone out whereas the X5 has a dedicated line out and is far superior. The firmware support on the AK was non-existent by the time I sold (don't know if that's changed since) and I was experiencing semi-regular lock-ups that required a full library re-scan which took forever! Oh, and there was a bug with random playback that played the same songs all the time.
The X5 is larger, not quite a sexy or sleek, but is a joy to own and use. It's not without its flaws as you'll see in this thread, but there is constant effort and communication from Fiio trying to fix it and they're already in a few months ahead of where the AK100 got to after >1 year. Personally, I haven't even come close to regretting making the switch.