Open Cart is a solution for small businesses, but if you are setting up a business with the prospective of growth and taking it to a higher level - Magento is the best solution, because it has a big number of features available out-of-the-box, furthermore it’s easily extendable with a variety of free and paid extensions that you can find on Magento Connect.
There’s much more to add about its advantages. In a nutshell, you get complete control over SEO settings, nearly unlimited control over the way your shopping cart will look, get access to order history, product comparison, etc.
If you got interested in Magento platform - I recommend you to look up our Magecom_dot_net blog posts named "Magento Features Overview" and "Magento 2 features overview. What's new for online merchants?".