Although i feel that amazon might be easier to promote, clickbank definitely has a higher payout. I guess it all depends on how you want to drive traffic to your site/list/landing page/whatever.
There are a lot of people out there making a killing through promoting clickbank products via e-mail lists. And there are also a lot of people who have a passive income through amazon.
It virtually comes down to what interests you more. Do whatever you think you'll have more fun with. Amazon may take a little longer to start seeing results because generally, you have to wait until you start ranking in the serps before you start seeing any sort of money trickle in.
Lorenz