Annual is the cheaper option: as the ~2 months free badge on the Annual button says, an annual period saves you roughly two months' worth. The exact figure appears on the plan card and on the Review your order screen. Monthly wins on flexibility instead.
You choose the period on the Plans & pricing page. The Monthly and Annual buttons under the heading switch it, and the Annual button carries a ~2 months free badge. With Annual selected, each plan card shows the price as a per-month figure with billed annually underneath. Before you pay, the Review your order screen spells out the Billing period and notes that it is "Billed as one annual payment".
Monthly billing means no single large charge, and a short commitment you can revisit often. You can also start on annual and switch back to monthly whenever you want.
Plan features are identical either way — paying annually unlocks nothing extra, it only lowers the total cost of the same plan. When you change plans, upgrades apply immediately and downgrades take effect at the next billing period. MarbleMap takes no cut of any sale; revenue comes from subscriptions only, so your billing period does not affect your transaction costs.
Also asked as: Is there a discount for paying yearly? · Should I subscribe monthly or annually? · What do I save on an annual plan? · How do I change my billing period?