How do I choose between white and cream paper?

A style choice with one technical consequence:

• Cream paper — warmer, softer, the convention for fiction, memoir, and most narrative nonfiction. Easier on the eyes for long reading.
• White paper — brighter and crisper, better for nonfiction with diagrams, workbooks, children's books, or anything with photos and illustrations.

Cream stock is usually slightly thicker than white, so your choice nudges the spine width. That's why the Cover Maker asks — set it to match what you'll actually order from your printer so the spine comes out right. If you change it later, re-check your spine.