There are two Paypal options available in the dropdown under Setup > Client Payments: Paypal Checkout Express and Paypal Payflow Pro.
Paypal Checkout Express correlates to any of the options at https://www.paypal.com/webapps/mpp/compare-business-products. If you're unsure which type of Paypal account you have, it's very likely you should select 'Paypal Checkout Express'. Note the transaction fees on the Standard option are the same as Stripe at 2.9% + $0.30 per transaction.
Paypal Payflow Pro correlates to the offering at https://www.paypal.com/webapps/mpp/payflow-payment-gateway. Payflow is a gateway that requires you to have an underlying merchant account, so setting this up is a much more involved process, but the resulting rates will be lower. (Please note that when signing up for a Paypal Payflow account, your email address used on that account will need to match the email address with which you log into Full Slate.)
Please note that if you have a Paypal Checkout Express account but have selected Paypal Payflow Pro under Setup > Client Payments, you may receive an error message like "400 Bad Request. Unsupported HTTP method." in the client booking process. To fix this, simply select "Paypal Checkout Express" in the dropdown at the top of the page at Setup > Client Payments.