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. Clients with a credit plan, or with a monthly subscription with a Bronze Plan (60,000 emails/month) or higher, have unlimited access to all the features and functionalities of SendinBlue.
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.