It mostly depends on your boat's transom height. Sometimes, your low profile transom wouldn't allow for any other type of bracket than a flat bottom one. Flat bottom bracket's are simple to make and typically take less material, so they are more economical. With high flotation brackets you get a higher volume airbox and a wider, more distributed load-transference surface for your transom to carry. A higher volume airbox could help or completely compensate for the addition of the outboard's weight at the most leveraged position on your boat to give it a flatter at rest attitude or correct a previous bow high one. A high flotation bracket's geometry tends to be better at supporting high torsional/torque loads and the 'vee' bend in the bottom of their airboxes lends itself for better longitudinal load carrying.