The pope, while certainly capable of the occasional ill-advised ad lib, is still protected by an institution with centuries of experience in diplomatic nuance. Nobody need worry about a gaffe coming from a meeting orchestrated by the Vatican Secretariat of State. 

Trump, on the other hand, is the leader of the gang that couldn't shoot straight. His administration can't get through a morning, let alone a whole day, without two or three juicy diplomatic snafus. And, unlike the pope, Trump's spontaneous comments are almost always unkind and self-serving.

So, if a diplomatic flare-up results from this meeting, you can bet your bottom dollar that the one responsible will be the guy with the comb-over, not the one with the beanie.