Do you never re-read titles? If so, then rentals to supplement the prepub streaming makes sense.
If you do re-read titles, then using your credit to buy a book is a good deal too.
For purchases, one thing to look at is the bonus content that is not included in catch-up months since those are the prepubs. It varies, but some of the content is really good -- my favorite is the bonus stories in the Realist Hero premium edition, where they aren't what-if or silly bits, they're nice additions to the main story.
Bonus list - https://forums.j-novel.club/topic/4902/premium-e-book-bonus-contents-list-updated-march-06-2023/21