Democrats have an easier time regaining control of the US Senate in 2020 with Biden.

Democrats will win every Democratic held US Senate seat up for re-election in 2020 except AL(Jones-D)
If Sanders becomes the next POTUS, his VT US Senate seat will temporary go Republican with current Republican Governor Phil Scott picking Sanders VT US Senate replacement. Whoever Governor Scott appoints will lose in the 2021 special election to the likely Democratic Nominee- Peter Welch.
45 Democratic US Senators with Sanders/46 Democratic US Senators with Biden.
Democrats will need to win 5 Republican held US Senate seats up for re-election in 2020 to regain control of the US Senate.
CO(Hickenlooper-D) defeats Gardner-R.
AZ special(Kelly-D) defeats McSally-R.
NC(Cunningham-D) defeats Tillis-R.
ME(Gideon-D) defeats Collins-R.
GA special(Warnock-D) defeats Loeffler-R or Collins-R.