It is better to sell on Amazon, Shopify, or both depending on your business model, your marketing plans, and your business goals since they are really two very different things. Amazon is a marketplace where you can pay a fee to sell. Shopify is a website platform where you can integrate a shopping cart, inventory, orders, design, and customer management. Shopify is a bigger commitment but offers more freedom & versatility. You can also sell with both places if it makes sense.