I haven't used clickbank but I guess Google Adsense is much better when it comes to profitablity. I've used Google Adsense for my forum and yield good results. Though the money I generated is not that big, given the fact that I used Adsense for a short period of time, it's not bad at all. It's hard to earn in Adsense but I am surpirse to earn a bit from it in a short period of time.