It all depends on the frequency and volume of emails that you want to send.
If you wish to send emails regularly and have a large contact list, a monthly subscription would be the best option for you. On the other hand, if you don't send emails often then the credits would be a better option; as they will never expire.
Don't forget that you can change or cancel your plan at any time. You can start with a monthly subscription and if you see that a credit system would be best for you, you can change plans without problem. Once you are ready to send emails regularly, you can go back to a monthly subscription.