Depends upon the teams involved and the way the game is going.

If the other team’s shooters are more accurate than ours, I’d play the WD to interfere with their opportunities to score. If my shooters are more accurate, I’d play the WA to give us as many scoring opportunities as possible.

Then it also depends upon your players particular strengths and weaknesses.  I have one centre court player who is a great WA, really good at getting away from her defender and moving the ball around and into our goal circle. But she’s not good at defence, can’t keep up with who she’s defending and gives away penalties. So, she plays WA, even if we’re short. Two years ago, I had a WD who was excellent at reading the play and she could intercept a pass even before the other team’s WA knew the pass was coming.  So, she would play WD, even if we were short.

Bit, everything else being equal, if I’ve got 6 players, I prefer to play the WA and leave WD vacant, because I want to the team to think like an attacking team.  Leaving the WA off and playing the WD can get them thinking like a defending team.