I have the same question as you are, quite frankly 400$ is a bit of a stretch for me, like u my headphones are shure se 310, and just now I have placed an order for the fiio x3 and was comparing the x5 so here is my thoughts if its of any help to you.
x5 has dual core ( no essential its not a smartphone )
x5 has bigger ips high res screen ( also not essential its suppose to be a music player)
x5 wheel poses a question mark on durability ( nothing can beat physical bottoms, more moving parts more risk to break down)
x5 has PCM1792 chip, I really have no idea if it`s better or not, but to be honest you can only listen to judge.
x5 has 2x amps of LMH6643 again I don`t know if it`s better or not but if better then by how much.
x5 seems to have less battery life according to the reviews.
x5 doesn`t have internal storage so you have to increase like 40$ to get 64gb microsd, so add to the cost while the x3 fully rigged tops at 200$
x5 is bigger and heavier if you do sports this will pose a problem, x3 is relatively smaller, has smaller screen so better battery life.
despite all that I am just like you and couldn`t find any direct comparison between the two, if you are in U.S buy one, try it and see if it`s worth it, if not return it.
because honestly speaking you have to consider the concept of diminishing returns, how much would you be getting for almost 230$ more, and think seriously about the durability of the ring, how much of better sound quality will you get and would you even notice?, and would your headphones actually be able to output ?, now I would understand buying an x5 if someone owns a se5xx series thats costs 500$ but for a sub 200$ earpuds I don`t think it will make much of a difference.