What is better SEO or digital marketing?

SEO and digital marketing are two separate but related things. It’s important to invest in both of them in the way that works for your business. If you’re starting from scratch, many digital marketers focus on testing first and determining which channels and combinations of channels work best for specific industries and business offerings. Digital Marketing includes things like Pay-Per-Click Advertising, Social Media, Search Engine Marketing, Email Marketing and so on. If you’re only investing in SEO or only in Digital Marketing, it’s not going to be as effective as considering both together.
SEO is about being found in search engines and this component can be split into two types of SEO: SEO to drive ranking for brand keywords and SEO to drive ranking for unbranded keywords. 
Branded keywords are things like your brand name, these are usually further along in the conversion process compared to unbranded keywords which are broader and more exploratory searches, think of them like general industry keywords not specific to a brand name. Both of these components are important to a strong SEO strategy. However, to get the most out of your website it’s also important to layer digital marketing on top of SEO as this will multiple your results over and over.