It might seem obvious but here’s why you should always motor-sail if you can.
The most amount of energy is taken up getting your boat to move, just move, that initial push through the water absorbs a lot of energy. There’s not a lot of friction at that point but you have to start the displacement moving water out of your way.
If you have enough wind, True or apparent that can have you sailing, even at a very low speed, then you have enough wind to overcome that initial displacement hurdle.
If you can do that then the power you add via your engines goes directly into overcoming friction and the mounting resistance from that point forward.