Authorize.net is a gateway to a merchant account, so you need a merchant account in order to use it. Paypal, can be used in the same way if desired, but ads the ability to accept credit card or Paypal user payment without the need for a merchant account.

Paypal is great if you are starting out, as getting a merchant account can sometimes be difficult. Many people see it as being more professional to have a merchant account and something like Authorize.net because customers can complete their transactions without leaving your website.

Also be aware that with a simple Paypal setup, no SSL certificate is required, however if you choose to go the merchant account route, either through Paypal or some other gateway provider, you will require and SSL certificate.

I hope this helps.