The verdict
For most small B2B teams with a developer, I'd pick Stripe. It is the safest first choice, it grows from a first payment to a platform, and your accounting tool and CRM probably already talk to it. If you sell software worldwide and do not want to deal with VAT, GST and sales tax in each country, pick Polar.
Do not choose on price alone. Both charge per transaction, and the cheapest option changes with your volume. Polar also has paid tiers that lower the rate. Work out your own sales mix first. If you are unsure whether you will want to be the seller later, remember that moving customers and subscriptions out of either tool takes effort.