Subdomain or subdirectory - Best for seo?

I need to install a blog on one e-commerce website but don't have access to the root. I can make a subdomain such as blog . domain . com or a sub directory such as www . domain . com /blog/

Are both theese urls worth the same in a seo perspective? I mean if my domain allready has high authority? Or is a subdomain harder/easier to rank?

Any suggestions are welcome

Thanks!!