Having been on a 2 different D3 Quests that were on both ends of size due to my own weight fluctuating so much, here are my experiences.
Turning when in perfect shape coming into the buoy makes little to no difference provided you've tweaked your bindings and fin to suit your normal style of skiing.
When you come into the buoy too fast and/or narrow, the shorter will be easier to get around the buoy but harder to make a smooth/clean turn so you will have to exercise some extra patience. The larger ski will want to keep going and requires more effort to slow down but will make a cleaner turn without wanting to stop. In this case, the shorter ski is more cooperative and forgiving IMO.
When you get to the buoy really early, the shorter ski will be much more inclined to break you at the waist BEFORE the buoy because it starts to dive down into the water. The larger ski will keep you moving and will be more tolerant of poor handle control. In this case, the longer ski is more forgiving.
Binding placement and wing adjustments can help bring the two ski's more in line with each other but inevitably, one will work a little better for you than the other depending on your style and your level of fin tweaking ability. IMO, if your fin tweaking skill is lacking, you're better off with the larger ski since the stock numbers will be closer to what you need.