BigCommerce is a great ecommerce platform - we've built some awesome sites using it. But it is no Shopify.
As a business, Shopify is more that 10xee bigger than BigCommerce and with this size comes an incredible amount of investment in development. Shopify just gets better and better, as new features and tools are rolled.
What this means for a merchant is that if you want to use the latest and the best ecommerce platform it has to be Shopify.