Egg yolks vs Whole eggs
If you want soft and moist, you need egg yolks. Their emulsifier and fat content makes dough pliable, soft and smooth, and retains moisture. 

Egg whites dry out a dough. This is sometimes desirable, e.g. in pate a choux. Eclairs made with whole eggs often have wet planes in the middle, resulting in an underbaked impression. If you remove some yolks from the dough and use a mixture of whole eggs and egg whites, you get a firmer, drier dough. But in many cases, people want softer, moister end products, and in such cases, recipes which increase the ratio of egg yolks to egg whites (by using more yolks than whole eggs, or yolks only) will give you a better result.