Luckily for you, there definitely exists a rule that tells you the best possible move to play for every given configuration of pieces—the rule that tells you the move that maximizes the probability of victory (or since draws exist and may be acceptable, the move that minimizes the probability of defeat.

If your opponent is a perfect player, each move has a 0% or 100% probability of victory. You can only maximize it in a trivial sense.

If your opponent is an imperfect player, your best move is the one that maximizes the probability of victory given your opponent’s pattern of imperfection. Depending on what this pattern is, this may also mean that each move has a 0% or 100% probability of victory.

Allowing A and B, and allowing an experiment on A vs. B, may create different incentives, and these incentives may be different enough to change whether we should allow the experiment versus allowing A and B.