No way in hell would I want "credits" over cash.
Also, be sure to lock down your coupon codes....guys like me can and will take full advantage of it (not in an illegal way, just in a way that we make sales without providing any useful content.....basically arbitraging off of your brand name).
Well, I should say "old me".
Also, be prepared for a 3-6 month build up/learning curve....depending on the vertical you are in. Very rarely do you see immediate profits and sales when you start an affiliate program, despite what these affiliate managers tell you.