Standard Distribution plan is good if

• Your book is not in a price-sensitive market, or
• You don't mind lower royalties to keep the price low, or
• You are experimenting with distribution and not yet ready to make upfront investment in inventory.

Premium plans are good if

• You want to lower the price of your book, or
• You want to earn higher royalties, or
• You are sure of the book's market potential and don't mind investing upfront to increase your returns.

The choice between Premium - 200 and Premium - 600 depends on what your price and royalty goals are and how much upfront investment you are comfortable making.