I've done a lot of checkout integrations and The Stripe API is orders of magnitude easier to integrate than the PayPal API. As the author suggests PayPal might be more popular because not everyone has a creditcard. Another factor that helps PayPal is that you don't need to have your card at hand to make the payment, if your account is active, the only thing you have to do is click "next" 2 times and you're done.
Shameless plug related to this post: I'm integrating both Stripe and PayPal in my software licensing SaaS product where I intend to provide the nuts and bolts related to accepting payments and handing out and validating the license keys for a standalone software product. It's still a Work in Progress, the landing page can be found on https://license.io