Your account can use both models at once - they're not mutually exclusive:
A recurring monthly or annual fee that raises your limits across the board - more surveys, more questions per survey, more responses per month, plus feature capabilities like QR codes, email invitations, scheduling, and exports. Best if you run surveys regularly.
A one-time purchase of a fixed number of response credits, spent only by surveys you've explicitly set to the Pay-per-response type. Best for occasional, one-off research where a recurring subscription isn't worth it.
Manage both from your Billing page - see your current subscription, its renewal status, and your remaining credit balance in one place.