Niche site store.... subdomain or new domain?

I run a content site that is basically the top site in its niche. Currently we're adding an ecommerce store, powered by Shopify. My first instinct is to create the store at shop.currentdomain.com. However, I could also register a new site, something like [nichekeyword]swag.com.

Does it make a difference from an SEO perspective? Subdomain builds on authority of current site, so unless the new domain had a vital keyword in it, subdomain is better right?

Talk based on personal experience. Use sub domain. It can leads a sales without doing any promotion. The main sites leads additional 15% sales.