FUN and NFB-12 -should- sound on par with each other (I say should because nobody owns an NFB-12 yet, there's been delays on shipment). Their amping power output is the same.
Both of them have USB input. The FUN has more connections for speakers/extra amps, and an analog line-in.
Basically, the DAC section in the NFB-12 should be better because of the dual WM8741 chips and the NFB-12 is smaller. The FUN's advantage is the extra inputs/outputs, and swappable OPAs to customize the sound.
If you don't need the extra connections and can wait until Feb 15th (yes, they're that backed up on NFB-12 orders, plus chinese holidays) then the NFB-12 is for you.
If you want ONLY a headphone dac/amp (no speakers or anything), then look into the Audio-gd Sparrow. Version B with the WM8741 added in is the most cost effective, and will sound similar to the FUN and NFB-12 (all 3 of these have similar outputs and internals)