Neither of those trumped-up -- ahem -- reasons get at Trump's real motivation for taking a pass on the debate. He let on what that was during an interview with Fox's Bill O'Reilly on Wednesday night.
Donald Trump says on Fox News that six debates are enough: "We're going to go on forever with these debates."— Jenna Johnson (@wpjenna) January 28, 2016
Trump is sick of debating. Period.
He also knows two things:
1. He is not a great debater but probably had his best performance in a debate in the last get-together of the candidates. (His amazing statement announcing he would not participate in the seventh debate included this line: "There have already been six debates, and according to all online debate polls including Drudge, Slate, Time Magazine, and many others, Mr. Trump has won all of them, in particular the last one.")
For Trump then another debate this close to the Iowa caucuses has almost no upside. His attacks on Ted Cruz are working. All of the second tier candidates are either attacking each other or Cruz. Thousands of people are coming to every one of his rallies -- including the one he will hold tonight in Iowa while his rivals debate. He is getting wall-to-wall media coverage and will continue to do so.
What Trump wants to do then is run out the clock. Take as few risks as possible between now and Monday. He and his campaign know that if he wins the Iowa caucuses, he will almost certainly cruise in the New Hampshire primary eight days later. Win those first two states and Trump starts to look (even more) like a juggernaut for the Republican nomination.
That's why Trump isn't debating tonight. The rest is just a smokescreen.