When to leave the 20 point?
The question is more generally about when to leave pieces in your opponents home board.  If those pieces are safe, you can theoretically leave them there until all of your other pieces are in your home board.  The question is, should you?  The advantage of leaving these pieces is threefold.


It forces your opponent to play safe when bringing pieces towards or into their home board, as you have a credible counterattack sitting there.  Without that, they can take a much looser approach to their home board and you can only punish them if they hit one of your pieces.
It prevents them from completely blocking their home board, meaning you can always (theoretically) bear on.
It delays the beginning of the pure race.


The disadvantages are:


Those pieces can be much harder to get to your home board.  This is much more of a consideration if the pieces are sitting on the 24 point rather than the 20 point, as it's very easy for pieces on the 24 point to get stuck.
You can be forced to make a disadvantageous move if those are the only pieces not in your home board.
It delays the beginning of the pure race (this is a downside if you are ahead as it gives your opponent more chances to turn the game around).


In the case where you have already moved both pieces to the 20 point, disadvantage #1 is less of a concern.  Consequently, whether you want to move these pieces as opposed to others mostly boils down to whether you are ahead or behind: move them if you are ahead so that you can start the pure race earlier (which solidifies your lead) and leave them if you are behind to try to disrupt/hit your opponent and gain an advantage.

Necessary caveat: all of this depends on both the rolls you make and the situation on the board.  If you have a roll that lets you safely advance pieces from one of these positions but not the other, this can force your hand.  Additionally, if you can hit a piece in your opponents home or outer board, that should take extremely high strategic priority.