The main difference between the Unlimited International Plan and the $10 Pay As You Go card is the pay structure in which you make the international calls.
The Unlimited International Plan includes unlimited calls to 60+ countries, including mobile phones in Mexico, Canada, China and India. These plans also include Roaming in Mexico at no extra cost. Regardless of how many calls you make or what countries you call, you will be charged the same set fee.
The $10 Pay As You Go card, on the other hand, will charged various rates based on location being contacted and the duration of the call. Remaining card balances, however, do not have an expiration date and will carry over to your next service cycle.