Biden has a stutter, which he often finds difficult to control. This often results in his brain misfiring when he speaks.
Trump is just an ill educated buffoon with a limited vocabulary that he has a mastery of. He's used mob tactics all his life. A playground bully with influence and money.
It’s absurd to say that Trump is not gaffe prone and to say he is articulate is deranged, I have no fucking idea what @BlueMoonAcrossThePond was thinking when he said that but we can very quickly find 1000 examples of him spraying out word salad because he can’t articulate his thoughts.