Clue: it isn't because Mueller is friendly with Comey.
“So if I say something and he (Comey) says something, and it’s my word against his, and he’s best friends with Mueller, so Mueller might say: ‘Well, I believe Comey,’ and even if I’m telling the truth, that makes me a liar. That’s no good.”
Trump opens his mouth and lies flow out.
Comey, on the other hand, as FBI head, took notes.
I would believe Comey over Trump because he comes across as valuing the truth.