I would recommend Stripe because it has a feature called Billing which is built for Premium plans. While adyen has more Payment Options, Stripe has lesser charging rates per transaction. But this also depends if your product is available worldwide or to specific nations. If you want to go worldwide, I would recommend Adyen as It is scalable. If you want to go for some specific nations Stripe will be better and easier to integrate but Stripe will also be great if you are worldwide.

Question for you: Where will you be running the transactions through? Will they be done in each country separately or done domestically? The reason I ask, Payfacs (i.e.: stripe, braintree, square, etc) charge two extra transactions if the processing is done domestically. The first being on an international card 1% and the second being in a currency other than your home country's currency 1%.

I'd recommend you check out Recurly. They are one of the leading solutions in the recurring billing space, but also support one-off orders. Recurly answers most common e-commerce use cases, will also give, out of the box, rich plan/subscription management functionalities to all the teams within your organization. They also come with great documentation, and SDKs, which from an Engineering standpoint, made it a very enjoyable pilot to work on, at the time. You'll also get great visibility/BI/analytics for free, allowing you to monitor the health of your business. Your Finance team will also get all of the data that they want. Without having you write any line of code. As you did touch upon integrations, they integrate with major payment gateways, including their own, support webhooks, integrate with Segment and therefore any tool that integrates with Segment, which makes this solution one of the most extensible one you'll find. Eg. Triggerring some Email Marketing "journey" (workflow) in AutoPilotHQ based on certain events.

PayPal is a dinosaur and a nightmare to work with, both in terms of API as well as on the business side. We support both because our A/B tests showed a material increase in conversions by offering both, but I would drop PayPal in a heartbeat if I could. Stripe is a joy to work with.

I prefer Stripe for developers' ease of integration and PayPal for providing the simplest user experience (i.e., improved revenue.) In my experience, neither platform requires a user register to pay via credit card. However, as noted, many people have a PayPal account, which allows quick and easy payments through a "more" trusted name.

I used Stripe and Paypal and settled on Paypal for the customer. However, the PayPal web hook back end is a complete mess (at least in PHP). Unless they have improved very recently, PayPal web hook integration is poorly documented and barely supported with bits and pieces of important code on GitHub and elsewhere. I can't speak for other languages. But given the popularity of Paypal (i.e., improved revenue), I decided to keep using Paypal after I got the web hook integration worked out.

Stripe is strong because of their security. It is difficult for the programmer to install it, and in my case was impossible since my hosting provider did not have up-to-date software that it requires. I had to use another program on top of it as the shopping cart instead of using Stripe's shopping cart. But Stripe integrates with everything if you can get it to install, and is stable.

The big reason to go with PayPal is the 6 months same as cash or multiple payments over weeks for your clients. Some clients need this and you will lose them without PayPal. PayPal may change their policies, so in my opinion, PayPal is not as stable as Stripe. Some clients refuse or can't use PayPal, where Stripe is available to everyone with a credit card.

In summary, I set up both for my clients. And I used Zoho for everything else - shopping cart, accounting, CRM, banking, etc. It integrates with both Stripe and PayPal and pretty much anything else you use.

Be sure to read the Terms of both Stripe and PayPal, and I think that will help you with your decision.