On several occasions I've noticed that the Aries can hold a course with less input to the helm (fewer degrees of rudder less frequently) than the human helmsmen aboard.
My Aries (previous boat) never worked that well running downwind. It was OK on the wind or reaching but not downwind.
I now use an AP and probably wouldn't bother with another windvane. My AP, once it has "learned" the sea-state, hardly moves at all and of course it is only using amps when it moves. we find it electrically very economical.
And for the OP, I wouldn't even consider an ocean crossing without a dependable form of self-steering even if I have crew.