When choosing between Leithold's and Stewart's Calculus textbooks, the decision largely hinges on the desired level of rigor. Leithold's is noted for its rigorous approach, while Stewart's is often considered more accessible for beginners. The discussion emphasizes the importance of having a solid foundation in proofs, suggesting that a textbook with good problem sets and clear explanations is essential. Alternatives like Spivak, Apostol, and Courant are recommended for those seeking a deeper understanding of calculus concepts, though they may be more challenging. The conversation also highlights the convenience of local purchases over online shopping, despite the broader selection available online. For those unfamiliar with proofs, supplementary resources like "How to Prove It" by Velleman are suggested to enhance understanding. Overall, the choice of textbook should align with the learner's familiarity with calculus and their preference for rigor versus accessibility.