It really depends on your design and your application’s complexity.
If you think you can stay within the core layouts of Storefront and some minor CSS and template changes would be enough for your website, then you should go with CHILD THEME.
But if you are planning on multiple custom templates and heavy customizations on front-end in that case I would suggest you go with your own custom theme, as it would be easier for you to maintain it in future and also won’t be bloated with extra unnecessary codes that would have otherwise came with the PARENT theme.