Two different products
A one-time number is made for a single verification: you get a number, receive one code and the number is released. A rented number is yours for a fixed time, 1, 7 or 30 days, and receives every SMS sent to it.
When a one-time number is enough
If you only need one code once and will never sign in with that number again, a one-time number is usually cheaper.
When to rent
Rent when you will need the same number again: signing back in later, keeping two-step verification working, receiving messages from several senders, or running tests over days. With a one-time number, the next code would go to a number you no longer have.
Extending
A rented number can be extended before it ends, and it stays the same number. Plan the length you need and extend if the work takes longer.