Both are very important and they actually complete each others perfectly.
SEO is a strategy on the long term, mainly if your website is brand new, you need to optimize it & develop content slowly, it takes time to get good & steady positions.
SMO/SMM can really help by waiting, if you're involved & do things well on your social media profiles/pages, you can attract a steady audience also. It's probably all you will have at first.
You can also more control the SMM, it makes you less affected by Google's changes of algorythm if you have another steady source of traffic.
I'll add that it helps for SEO and SMM links & visibility is a strong factor now.
To learn more about how to grow the audience & visibility of your business online, read my blog.