Well, AdSense vs CPA is a battle that has been waging for a long time.
It comes down to your audience and niche. Let's say I make a blog about the xBox 360 - obviously I would want to get into Affiliate Marketing. I could link to various games for the console, accessories, etc. The people viewing my blog are likely to be interested in making a purchase, and those that are interested will be presented with relevant items that they need and want. If you're looking to open a site on say... 'seasonal allergies' there aren't many options for affiliate marketing there.
Basically, you want to judge whether or not you can provide relevant affiliate marketing advertisements and whether or not your target audience would be interested in making a purchase. You really have to judge something like this on a case-by-case basis.