The verdict
If you sell subscriptions and want a mature, broad service with recovery and analytics tools, I'd pick Paddle. If you are a developer or small team selling licences, subscriptions or usage-based services such as tokens or API calls, I'd pick Polar.
Polar's pricing model is easy to try, since it has a free plan and paid tiers with lower rates, so work out your own volume before you decide. Paddle charges per transaction and sends some cases, such as low-priced products and invoicing-heavy businesses, to a custom quote. Both set rules on what you can sell and review accounts. Check that your product and your country are covered before you build on either.