Both are commonly used by breastfeeding mums to help with milk flow, and the main reason some prefer sunflower lecithin is that it is non-GMO and a good option if you want to avoid soy, especially if there are any allergy concerns in the family. Since your lactation nurse already recommended soya lecithin for your specific situation, it might be worth going back to her to ask about sunflower as an alternative, just so she can advise based on your full picture. Either way, it is reassuring that you are looking into this carefully, and your PD or a lactation consultant can help you weigh which one suits you better.