I have not been able to find much regarding the difference between the two. What is the advantage of one over the other? Thanks.
Ad Manager's main point is to serve your own ad's. So lets say you run a website and have other websites that want to advertise on your site. You can set up ad manager to create a campaign for those advertisers. Ad Manager takes care of serving the banners and everything. Ad Manager is not PPC like AdSense. They have completely different purposes.
This completly depends on you and your sites there are many that make much more money selling ad space then what they would make from clicks on adsense so it depends on you and your sites if you have great sites that people would like to advertise on then there is a good chance you can make more money by selling ad space or if you have low traffic sites but you are able to find buyers for ads lets say you have a site that is PR6 but only gets a couple hundred or thousand visitors a month there is a good chance you could make 100 times more selling ads then placing adsense code on these sites or maybe not like I said it depends on you and your sites
If you are currently managing your own advertisement using an ad management software like OpenX, PhpAds, etc, You can use Google Ad manager to manage your ads.