Amazon gives you demand on day one but takes a share of every sale and keeps the customer relationship. Shopify gives you ownership but you have to bring the traffic yourself.
Start with Amazon if
Your product already has proven search demand, your margins survive marketplace fees, and you want faster validation than a paid-traffic build.
Start with Shopify if
You have an audience, content or a creator partnership, and you want to build a brand asset with repeat customers and email ownership.
The usual answer
Pick one, get it profitable, then add the second. Running both badly is the most expensive route.
