Over the past couple of years, Magento and OpenCart have emerged as the frontrunners in the ecommerce platform race. Both the platforms are under active development and are adding more and more extensions every day. But between them, OpenCart seems to be gaining more popularity among developers.
Magento has the advantage of being able to support multiple sites better and also has the most in built features. It is still the platform of choice for developers versed in the Zend framework. The framework, written in object oriented PHP, is supported by heavyweights Google and Microsoft too.
But OpenCart, has found a bigger following among new ecommerce developers. As mentioned earlier, Magento development has to be done in object oriented code. This structure, while making it easier to modularize and add features, makes the code API driven and harder for a new comer to understand, learn and code. In a continuously changing web development environment, new developers are time constrained and look for faster ramp up times.
OpenCart, supported by a vibrant and active open source community, has a shorter learning curve. Developers are finding it easier to adapt to the CakePHP framework as it is built on Ruby-on-Rails ideas. And slowly and steadily, OpenCart is also adding higher quality templates and extensions to match that of Magento.
Also, Magento and similar ecommerce platforms have been known to be demanding in their server and hosting requirements. OpenCart is a far lighter platform and thus needs slower and less expensive resources to run. Based on these pros and from the evidence of the growth shown by OpenCart this year, it does seem poised to take over as the dominant ecommerce platform soon.