Same winds, same size board, one wave rockered, one bump n jump rockered.
Which planes sooner? On windpower alone, the flatter rockered board.
Which goes faster? That depends whether you are on a wave, what the wave and you are doing and on wind strength and direction.
Which catch's a wave earlier? Again it depends whether you are planing on windpower alone or not.
Now again, both well powered, it makes little difference. But WHY would you choose the bigger board? Because the wind is not quite there, and you're UNDERpowered. Underpowered by wind, this does not mean you are underpowered once you get wave power as well.
Light wind wave sailing (the french call it surf-sailing, which I think is a good name) can be quite tricky but with the right equipment and conditions can also be really good fun. Ideal is a point break with a channel down wind and cross to cross of winds and a big wave board that turns well. I tend to choose sail size in those conditions depending on the amount of people out. Remember that once you are on a wave it may give more speed than the wind and the sail will start to create drag so a smaller sail may be faster. Reducing your straight line speed by making more turns reduces this drag as well.