which headphone is better upgrade?

Only you can decide which one you'll like the best
If you get either of them you will most likely need an amp...well you would nedd one for k701 which is a very inefficient headphone.
If you're sticking with the Fiio (you should it's a great little dap) then I would much rather recommend that you go with the Hifiman he400s, which is easier to power than the he400i. Sound wise they're close to each other, but the latter most likely needs more juice than what the Fiio is capable of delivering.
I have the he400s btw and it plays wonderfully straight out of a phone...and my bet is that a planar headphone will leave ypu more impressed than what the AKG will do...but it's your ears.

Edit: looking through the specs you may just be able to drive he400i with your current Fiio dap. If you have a stereo receiver/amp with a headphone out you can try that and then compare with the Fiio. If the stereo sounds better it'll likely be because it drives them properly...and then it is entirely up to you whether you want an amp or not:)
Personally i'd just go for the he400s and never have to worry about amplification.

Edited by Guldbamsen - November 11 2018 at 06:36

LCD X or Xc's .