Sri Aurobindo and The Mother never gave any outer (or even inner conditions) for chanting Their Names or the mantras. Even Sri Krishna does not give any such conditions. In fact he goes on to say, – and Sri Aurobindo confirms this, that even a wicked and fallen man chants God’s name he is soon liberated.
All this is the fabrication of the pundits. Perhaps they may apply to certain deities who may demand certain outer conditions for them to act. But the Divine is everywhere and in everything. All that He needs as a point of support is faith.
Both the mantras you mention invoke the Hoghest Divine whose very invocation purifies and redeems. In fact one can call their name anytime.
As to choosing the mantra, it should be that with which your inner being spontaneously connects. It is best if it is short and represents to you or brings to your mind Their image and Presence.
It is perfectly fine to remember Sri Aurobindo sometimes and the Mother at other times or both. A certain degree of plasticity is helpful in spiritual matters. In any case the Mother and Sri Aurobindo are one and the same in two bodies. One can very simply say Maa Maa Maa Maa Maa Maa Maa and that is enough.
Affectionately,
Alok Da
