The calculation
Discount equals one minus the annual price divided by twelve times the monthly price. Every presentation converts. Two months free means paying ten months for twelve, so the discount is one minus ten divided by twelve, or 16.7 per cent. A stated fifteen per cent off is simply 0.15. A headline annual price is converted by dividing it by twelve times the monthly rate.
A worked comparison
Take a monthly rate of 1,000 per user and an annual rate of 10,000 per user. Over twelve months, the monthly route costs 12,000 and the annual route costs 10,000, so the saving is 2,000 per user, or 16.7 per cent.
The cash timing is the other half of the picture. On day one the annual customer has paid 10,000 while the monthly customer has paid 1,000, so the annual customer is 9,000 out of pocket. That gap narrows by 1,000 each month, and by month eleven the annual customer is ahead. Averaged across the year, the annual customer has roughly 3,500 per user tied up earlier than they otherwise would. Saving 2,000 in exchange for that is a high implied return on the cash, comfortably above the cost of ordinary short-term borrowing.
The decision rule
For a buyer with available cash, annual prepay is usually good value on these numbers. For a buyer who would have to borrow expensively, or who has better uses for the money, the flexibility of monthly payment may be worth the extra sixteen per cent. The point is that the question has an answer, and the answer comes from comparing the discount against the cost of the cash rather than from a general preference.