I just recently switched from PPC to SEO.
-Both require full time attention in order to be competitive.
-SEO is 20x bigger than PPC. (95% vs. 5% click share)
-PPC can ban/suspend your accounts and not allow you back. (happened to me when I was being legit)
-PPC you need to float a lotta cash to keep your campaigns going.
-PPC - the ad network is trying to make as much as possible off the traffic. This makes it difficult to make a high profit margin. They'll charge more if they see your doing really well with the traffic.