The verdict
If you run a Dutch or Belgian business with client invoices, expenses and VAT to file, I'd pick Moneybird. If you sell software, subscriptions or usage-based services internationally and do not want a tax project before your first customer, I'd pick Polar.
A small software company in the Netherlands could use both. Polar collects and pays out, and Moneybird holds the books. Because Polar is the seller of record, check how its payouts and invoices reach your bookkeeping. Ask your accountant what they accept before you decide.