for some ppl image ads work best and for some ppl text + img both give good result. We can earn more with refferal links but its hard to put ppl in the buying word (its what we do in cause of AP) If ppl will purchase product then u will give good amount and if not then u will lose one visitor (coz u need 2 clicks) on the other hand, adsense for content need just one click and u will paid. So result is, adsense for content works well but if u r in AM then u can sell products very easily In my case, adsense for content is best.
It completely depends on the kind of website... some websites make more money with even affiliate programs than with adsense But IMO adsense for content is working better and a 300*250 gives the best CTR