Customer journey includes awareness, consideration and purchase stage. SEO and social media marketing has its own impact in each stage of the customer journey. For awareness stage, brand needs to get more exposure as much as possible and reach mass audience in order to capture the largest business opportunities. To cater this, a channel can help you reach more potential audiences are better. Make use of social media marketing let you engage with customers, whether they are browsing, watching video, sharing or finding something in social media. When you engage, people will be more familiar with your brand, preparing for the next stage, consideration. When come to consideration, social media marketing also functions well. With retargeting, you can show ads in social media who already have interest in your product or service. These people are more willing to buy your products as they already know your brand or familiar with your product. Besides social media, SEO can also be adopted in this stage. When someone is in need with your product, they may search it in the search engine. When your site is ranked high in search engine, you have a greater chance of being seen and visited. In the purchase stage, SEO will help the most. People have higher chance to buy something which they actively searching instead of seeing an ad in social media. Similar to the above, good SEO ranking makes your site higher visibility and higher chance of visit, capturing the opportunities of purchase intent. 

For SEO, you probably need a SEO agency as SEO is easier said than done. It involves a lot of technical skills, such as keywords selection, fixing technical problems and content optimization. A SEO agency can help you with all the above. On other hand, social media is easier to operated, not necessary like SEO to hire a SEO agency to work for you.