Sorry to hear you had such a bad experience with Clickbank. Refunded 5 weeks later? That’s pretty extreme! I could not find any info on CB on whether gravity takes into account refund rates, but from other blogs, I read that NO, it doesn’t take it into account. The Earned/Sale category however does take this into account, which is why the average $ earned per sale will sometimes be different from the actual commission you make.
I’d stick with Amazon if I were you. All of my Amazon sites only have Amazon affiliate links. If you use Google Adsense display ads on your site, you’re literally taking people away from your site for the sake of just a few cents with these type of ads. If you direct them just to Amazon, then you have a greater chance of earning more money from that click.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.