Both give you the same credits with all features included — the difference is how you buy them.
Unused posts never expire with either option — plan posts carry over to the next period. You can have one monthly plan at a time and still buy extra packs whenever you need more. Each card on the pricing page shows the price per post, and plan cards show about how many posts a day you get.
To estimate your need, add up the AI posts per day of all your campaigns: for example, 6 campaigns posting every 2 hours is about 72 credits a day.
Start with 25 free posts — no credit card needed.