It depends. Are you related to the children that you are taking guardianship of? Are the parents rights being terminated?
I say it is always better to get permanent guardianship. If you feel later on that the parents are ok to have the children reunified with them, and the CPS case is closed, then you as the Guardian, and the parent can petition the family courts on your own to terminate the guardianship and have the children returned to the parents. If you never feel it is ok for the children to return to the parents, then you don't have to do this. If the parents rights were terminated, then you wouldn't have to worry about this anyway, because the children wouldn't be reunified, even if later you thought it would be a good idea.