If you use PayPal Express Checkout, when your customers check out, they will be directed to a page that requires them to log into their PayPal account or create a new one. Therefore, this is the best option if you anticipate that most of your customers either have a PayPal account or will sign up for an account on checkout.
It may also improve sales; according to PayPal’s website, the Express Checkout button "can increase sales by 14% on average*" and has "conversion rates [that] are 23% higher than standard checkout at some of today's largest online retailers.**"
If you use PayPal Website Payments Standard, when your customers check out, they will be directed to a page that allows them to log into their PayPal account or pay by credit card without having to sign up for a PayPal account. This is the best option if you anticipate that most of your customers will not want to sign up for a PayPal account.
Why can't I use both options with Shopping Cart Creator?
If you try to enable both options in the Payment section of Shopping Cart Creator, you will get a warning message that says "Only one PayPal payment method can be used," which will then prompt you to disable one of these methods. Since one method requires that users sign in to PayPal and the other doesn't require users to sign in to PayPal, they contradict each other. Consequently, you can only use one method or the other.
Which one should I use?
PayPal Website Payments Standard allows users the choice of signing in or not signing in, and can be considered the default choice; PayPal Express Checkout can be more efficient for PayPal users and may facilitate sales.
It may depend on whether or not you also have a credit card processing option; if you do, you can use PayPal Express Checkout, since the customer will only need to use this option to pay with PayPal funds. If PayPal is your only payment method, you may want to use PayPal Website Payments Standard so that your customers have the option of using a credit card.