The main difference between the two payment is that while unlike Credits, Subscription is a recurrent payment.
In contrast to Credits, Subscription requires regular monthly payments (the amount depends on the number of users you'll wish to involve) and comes with automated G Suite Data Sync. This opens you the possibilities of adding marketing content or performing any changes to signatures with no additional charge. If you know that you (or your marketing department) will want to use the signatures as a communication channel, Subscription is the best pick for you.
If you want to set up your signatures and leave them be for a longer period of time, Credits might be the best option for you. You purchase Credits to your SignatureSatori account and exchange them for signature set up. If you want to set up 20 signatures, you'll need 20 Credits and you won't be charged anything more (unless you decide to redesign your signatures later). If you want to utilize the Daily G Suite Data Sync, there's an option how to combine it with Credits, although not as powerful as with Subscription.
There's always an option to combine both methods - many companies purchase Subscription for a smaller group of users who communicate outside company and keep the rest of staff on Credits.