I had a client who faced this dilemma of choosing between social media and SEO.
My advice is to do both when you can.
Because SEO is like the workout you do that makes you fit from inside and increases your lifespan.
Social media is like grooming or the clothes we wear.
Without a fit body clothes won’t look great.
So, if your site does not get a lot of traffic then first work on.
Also, don’t ignore social media totally.
Pick a platform where your customers hangout and build your presence there before moving to a new platform.
Also see your comfort with the type of content you’ll need on a platform.
For eg, text and live videos work great on Facebook, whereas visual content with carousel work great on Instagram.
So, kickstart SEO on day 1, and start with one social media platform before you move to another.