I know this is an impossible question, the future is chaos, but we can at least identify and discuss some key factors.
When I say "successful leftist revolution" I mean one that happens soon enough to prevent, or have non-negligible odds of preventing, the collapse of human civilization as we know it due to climate change.
In all likelihood I think we're fucked as far as climate change is concerned, but we should still try to improve our odds. There's no prize for correctly predicting that we're fucked.
I don't think either timeline changes the overall odds. Joe Biden being in office won't change our priorities. Moreover, in the list of things that will help advance socialism, 'more time' isn't very important. A revolution will happen when we organize around our material needs, and our progress towards that will not change whether or not Biden gets into office. A revolution will only happen if the left advances an actual alternative to capitalism based on addressing people's material needs.