PayPal Pro is 2 services:
PayPal Direct Payments - Customer does not leave site to process transaction.
PayPal Express Checkout - Customer goes to PayPal's site to perform transaction.
This gives the customer the flexibility to pay with Credit card or pay with their PayPal account.
Authorize.net is basically the same as PayPal Direct Payments in that the customer pays by credit card without leaving your site, but does not offer the choice of paying with a PayPal account.