Updated February 2023
In short: Salesforce has multiple tiers of its core CRM product — Essentials, Professional, Enterprise, and Unlimited. Each varied level of licensing enables different functions within the core platform. The Non-Profit Success Pack or “NPSP,” overlays on top of the core platform. So the question isn’t “do we have Enterprise Edition or do we have Non-Profit Success Pack?” They’re two completely different things that are complimentary to, not divergent from, one another.
Even further, you must be on the Enterprise Edition in order to install NPSP in the first place. As of 2023, Salesforce now offers additional Nonprofit Cloud pricing packages with additional capabilities.
And in case you aren’t aware, through Salesforce’s foundation, non-profits are offered a free product grant. That gets you 10 free Enterprise licenses of the Salesforce.com core CRM application, and the Non-Profit Success Pack can be added on top of that foundational layer.