Honestly I have heard quite a few of Fiio's in my day and i have never really found one that was worth buying. I got the oppo in and i was just in love. It isnt that bulky it is like having 2 smart phones in your pocket. Plus the oppo come with bands as do many portable amps to hold the two togethere.
The reason I became so set on the Oppo was because it has Reference A/B chip in it, aswell as the bigger desktop they use, uses that same exact chip. The price difference is like 700 bucks and the only thing you gain is a bigger amp that is more powerful but yet still sounds the same. So for $300 it is a steal. As well i just measured how big my Galazy S5 is with the oppo amp and it measured at 3/4 of inch thick. Now if you have a case on your phone that could change it alot im sure haha
, but with out it i dont think there is alot of pairs out there that when togethere are 3/4 of an inch thick.