Good SEO will give your site / content long term results for example top organic listings in search engines whereas PPC can be costly, the results are forced and you only have the results for as long as you pay etc.
I have handled sites that's just a few years old and had better listings than leading authority sites like Youtube, e-how and at times the BBC with what I call 'clever SEO'. With one of my clients sites we have had him in the 1 - 3 (top in the world) organic listings on Google for the past 2 years almost beating all other authority sites for his niche (dancing) and we have never spent 1 cent on advertising ... just good pure SEO and time / efforts