I'd say the difference is if you believe a Biden Presidency would give us a 4 year reprieve, while weakening the Democrats and ushering in a competent Fascist the next go around. Where 4 more years of Trump "could" lead to more and more incompetence paving the way for a revolutionary Democrat that can bring a Progressive utopia. You may feel it's morally just to not participate and let the chips land where they may.
That is accelerationism and it's failed, every time. People sat out 2016 because they thought if Trump won they could get Bernie in four years, how's that been working out?
The notion that electing Trump now will somehow head off a more competent Trump in the future is laughable, too. American politics are unfortunately cyclical, the next Dem president could win 2020, 2024 and do a great job and be replaced by a Republican just as easily in 2028. Clinton and Obama were generally regarded by the populace as good presidents, they put up hand-picked successors who both "lost." (Both won the popular vote, which is critical to be mindful of) Whether Trump wins or loses in 2020, a more competent alt-right president is inevitable down the road. In fact, I would see it as even likelier to happen soon if Trump wins again, as it would confirm to any ambitious Republicans that following in his footsteps is the key to success.
And frankly I'm not high on a theory that suggests wanton chaos and destruction is fine in the short-term if it'll pay off later in a glorious political revolution. Some of the most ardent Bernie supporters I've seen talk about Biden's healthcare plan like it would actively murder people, and then turn around and act like people literally dying due to Trump's incompetence is just a toll to pay to cross the Bernie Bridge.
(Not accusing you of saying or supporting any of that, just accelerationism in general)