I did some calculations between the two, and found that the concentration of fire in one shot worked out marginally better overall, like 2%, however my calculation lowered the expected damage of each shot by the probability that the opponent can use the brace token (or more simply put, brace adjusted damage).
However, I think from a practical perspective that the primary benefit of concentrating fire vs spreading fire is the opponent is far, far less likely to exhaust the brace against a weaker attack, and much more likely to be denied a brace when using a larger dice pool with more accuracies.
Therefore, it once again comes back to playing your opponent. Does he always brace to save 1 damage, 2 damage, ect. What is the damage threshold for her to burn the brace?
Why don't you just take the four damage from my front arc, as you know I will be throwing 7 dice at you during my next shot.