The client I mentioned joined authorize.net through their bank, so we didn't use the website. Weird.
On the comparison chart, "Total Monthly Fees" is how much you'd pay that month based on the dollar amount and number of sales you entered on the previous page. For example, if I enter $500 and 10 transactions, it tells me that PayPal would charge me $12.50, which is $500 at 1.9%, plus $0.30 * 10 transactions.
"Discount Rate" is credit card transaction "code" for what percentage of your purchase that they will charge you. "Transaction Fee" is an additional amount they will charge for each transaction, no matter how much it is. Obviously "Setup Fee" is a one-time setup fee for working with the company.
The downside of PayPal, imo, is that it can't be completely transparent as far as I know -- at some point you end up on PayPal's site, if only for a single page. Is that right, PayPal users?