I highly recommend Fermaid K, but I keep Fermaid O and DAP on hand as well.
Diammonium Phosphate (DAP) is just a source of nitrogen for yeast, nothing else, it is the simplest form of yeast nutrient and only provides nitrogen and phosphorus.
DAP alone does not provide the best conditions for clean and healthy fermentations.
The following is taken from Lallemand:
Fermaid K is a blended complex yeast nutrient that provides the full range of nutrients that yeast need to have a healthy and strong fermentation. Fermaid provides DAP, free amino acid, yeast hulls, unsaturated fatty acids, and other helpful products. .
Fermaid K supplements a series of important nutrients and bio-factors.
Thiamine, Folic Acid, Niacin, Biotin and Calcium Pantothenate