What's the difference between PayPal buttons and PayPal Express Checkout?
The second one seems more complicated than the first one to integrate in a web site. When I should consider to use Express Checkout rather than Buttons? What are the advantages?
PayPal Standard buttons are indeed quick and easy, but they are limited. One of the biggest issues with it is with regard to Guest Checkout, which allows non-PayPal account holders to pay with a credit card without creating an account.
With Standard buttons this is browser cookie based, so if anybody has ever signed in to a PayPal account using the browser in use, then the cookie will trigger and assume that the user will be logging in to PayPal. This causes the guest checkout option to be less prominent, and buyers often miss it, which results in lost sales.
Express Checkout uses the APIs which gives you more flexibility, including the ability to force the Guest Checkout experience if you want to regardless of any browser cookies. This can lead to increased conversion rates.
Another thing to consider is that with Standard buttons there is no guarantee the user will make it back to your site. Even if you have Auto-Return enabled in the PayPal account there is a delay, and the user could simply close their browser before they are sent back to your site. With Express Checkout the user has to return to your site before the process can be completed, so this gives you the ability to tie more post-transaction processing procedures into your checkout flow.