I wasn't aware of the Amazon FPS. I am going to check this out and see how difficult it is to implement it. I guess the thing with using Amazon is whether people have more amazon accounts than paypal.
I would use Amazon and Paypal. While Amazon FPS scales to micro/regular payment for you, the Paypal link above mentions opening one micropayment and one regular paypal account and switching manually between. Both are allowed to get the best rate and it is worthwhile if you sell from .99 past $10+.