â†' Magento is good for bigger stores with bigger budgets. Open cart is suitable for building your store independently and it is (almost) free to use.
â†' Magento's is relatively complex and requires programming knowledge and technical expertise. OpenCart is considerably easier to use & manage.
â†' Magento is occupying 19% of the current e-Commerce market.
â†' Magento is a resource-demanding solution so it requires a strong (and expensive) hosting plan to support your store performance. OpenCart is very lightweight so hosting expenditures will be less.

I've built a few sites for clients with OpenCart and it's really easy to setup and use. Opencart basic is free and very good, but there are many plugins to enhance it at very reasonable cost and the various plugin developers that I have had contact with are excellent people to deal with.

Magento looks very complex so for my clients I chose Opencart because of the shorter learning curve.