Both SEO and PPC are useful. In my opinion, ppc or cpc traffic can improve organic traffic. Take a new website as an example, it needs huge amounts of impressions and traffic to improve its popularity through advertising in search engine( Google, Bing, Yahoo,etc.). The disadvantage is once you stop advertising, maybe you can not find your site on any search engine. Therefore, you need SEO for the long term effects. Though, it will take you lots of time to post, update your information, blog and so on, it can give you long period value. What's more, if your website is good enough, the paid-traffic's visitors will remember your site and maybe become the organic traffic next time.