Have had the E12, but not the Cayin C5. I ended up with the JDS Labs C5...very pleased with the sound coming from my P7's with the JDS amp. I definitely prefer it over the E12.
Haven't upgraded my cable...but have "upgraded" cables on other headphones and found the "improvement" minimal or non-existent...guess I am one of the many skeptics on this front.
I definitely prefer the Cayin C5 and JDS C5 over the Fiio E12.
And if I had to pick between the two C5's :), I'd go with the Cayin.
Tons of power, very smooth, warm, detailed.
If looking to save some $$ and if form factor doesn't matter,
I'd go with the JDS O2.