It really depends on what is the purpose and for what time span you want this action to be effective. SEO is about optimizing your website to get higher rankings in the organic search results by using some tool, while SEA is a paid form of marketing. It is believed that SEA works well if you want to rapidly increase traffic to your website in short period of time. SEO is very efficient if you wish to work on the visibility of your website in the long term. Although usage of a combination of both activities would be optimal.