Which headphones are you planning to use.
The C5 just amplifies the iPhone output (going through the DAC and amp of the iPhone and double amping that signal) - while the Mojo takes the digital signal uses its own DAC and amp it's not really a fair comparison.
Anyhow - here is the testing setup:
Two iPhone 6 128 GB playing the same song. One with CCK and Mojo and one going into C5 (ignore the Fiio X5 strapped onto it) both go into the input switcher so I can flip back and forth in realtime - and I listen with my trusted Oppo PM-3
Differences:
Overall differences in short term listening are quite small.
Mojo is better at:
- slightly fuller, more rounded sound
- more impact, crispness to tones
- more "air" and treble detail
- a tad more natural sound
Overall, I was quite impressed how good the iPhone 6 / Cayin C5 sounded. Anyhow, just have the demo unit of the Mojo since yesterday night and charged it so this is all very new impressions about the mojo, while I know the C5 quite well.
So, is the mojo worth 3 or 4 times the price of the C5? Hard to say. Depends on your priorities. I would argue that for listening on the go, you might not need the mojo.
Second consideration - your headphones. sensitive IEMs are probably better suited for the mojo due to it's more refined construction. The C5 is a powerhouse in regards to drive even very large and demanding headphones with authority.
Cheers!