Both send through the same underlying gateway and the same account balance -- the difference is where the message originates.
The portal
A dashboard is the right fit for one-off or scheduled campaigns: importing a contact list, writing a message, and sending or scheduling it by hand. No code is involved. See Bulk SMS Tanzania.
The API
An API is the right fit when a message needs to be sent automatically from your own system -- an order confirmation the moment an order is placed, or a one-time verification code at sign-up. See SMS API Tanzania and, for verification codes specifically, OTP SMS Tanzania.
Using both
Most accounts use both: the API for anything triggered by an event in your product, and the portal for campaigns a person plans and sends directly.
