Regarding their reputations:

Paypal is probably what people associate with online payments. However, there are a LOT of people who have been ripped off or treated poorly by paypal or scammers. So while it has a broader reach (WRT users making payments), it also doesn't have as good of a rep as amazon.

Amazon is consistently at the top of customer satisfaction surveys. People don't generally think of them as a payment source, but this is just the current state of affairs. With their "passphrase" option, and other saas / iaas / services, I think the shift is swinging so that people realize amazon is everywhere.

I'd go with amazon. Paypal customer service SUCKS. Paypal support (even dev support, IME) SUCKS.

I've never had anything but happy thoughts about amazon.