The main difference is that unlike Credits, Subscription is a recurring payment - however, it has many advantages.
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.
Another advantage of Subscription is that it impedes the signatures from user "creativity" - all modifications done to the signatures will be reverted upon the synchronization.
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.