The verdict
If you sell software, subscriptions or usage-based services to customers in many countries, pick Polar. It takes the tax work off you and gives you billing logic you would otherwise build yourself. If you are a freelancer or sole trader who invoices clients and wants business money kept apart from personal money, pick Revolut Pro.
They can also work together. A solo developer could sell a product through Polar and receive the payouts in a Revolut Pro account. Revolut Pro is not a bookkeeping tool and it is not a merchant of record, so it does not take over your tax obligations on foreign sales.