Explain the difference between Nonprofit Cloud and the Nonprofit Success Pack, and how you would advise a customer choosing between them.

The difference is architectural, not cosmetic, and it determines almost everything downstream.

1. NPSP is a managed package layered on the classic model: Contacts, household Accounts, and donations as Opportunities. Nonprofit Cloud is built on standard platform objects delivered as part of the product — GiftTransaction, GiftCommitment, Program, ProgramEnrollment, FundingAward and so on.
2. For a greenfield customer I recommend Nonprofit Cloud, because it inherits platform capability directly and is the model Salesforce is building forward on.
3. For an existing NPSP customer the answer is not automatic. A move is a genuine re-implementation with data migration, integration rework and retraining. I would look at how much the current model is actually constraining them, what integrations exist, and whether there is appetite and budget for a project rather than an upgrade.
4. One hard constraint worth stating early: Person Accounts are not supported with NPSP. So the two constituent models are effectively mutually exclusive, which is why this is a re-implementation rather than a migration.