It's best to have both - as you had to provide proof of ID when you setup your bank account, linking it to your PayPal Account is also a good way to prove you are who you say you are - PayPal accepts this as they know you have been vetted. Linking a credit card serves as your backup funding source and additional proof of ID. Note, it's not mandatory as you can still receive money but, you may run into limits for sending money and withdraws - in order to avoid these limits, linking both a bank account and credit card removes those limits.
