It comes down to credibility. Each listener must decide who is most likely to be telling the truth. The track records of Comey and Trump, as well as on the record comments from each, paint a picture of their respective characters. James Comey has made decisions that allow for speculation on his judgement, but he has no history of lying. Donald Trump tells lies constantly. Trump's relationship with the truth seems incidental. When he says or tweets anything, his goal is to generate a desired reaction - the actual truth of the statement is of no interest to him. Truthfulness is a metric that Trump has no use for, or interest in.