In my opinion PayPal is the easiest and most reliable solution. I can't comment about which one will be cheaper though.
If you are in US and UK, I will strongly recommend offering Google Checkout along with PayPal as you will then be able to offer an easy solution to buyers who already have a Google Checkout account or ones who are not happy with PayPal for any reason. Also, their feestructure is really good, especially if you using Google Adwords.
As for Authorize.Net, they work with merchant bank accounts. Their support will be the best people to answer your questions.