I am considering going with either opencart or expression engine as an ecommerce platform for a small business. Is anyone able to comment on how these compare?
Yes. As a developer for 5 years plus, I would recommend staying clear from Both. You're best bet is wordpress or magento for Ecommerce. Magento is more for larger ecommerce stores, but either work.
Is there a specific reason why you've chosen those two?
Opencart is an Ecommerce platform whereas ExpresionEngine is a CMS. You'd have to use CartThrob or ExpressoStore etc to provide eComm functionality.
You'd probably be better off using Wordpress + WooCommerce or Magento (if you wanted to go self hosted) or use a hosted solution such as BigCommerce of Shopify
For a little more than 7 years we have had great success with WP / Woocommerce. Here is an example of a site using both: https://serenity-organics.com/. Both are intuitive and quite manageable.
I can talk about OpenCart. In short it's extremely unpredictable. Once you get it running, it runs, the thing is to get it to run.
That said there's a big community behind OpenCart, so there's support out there for it, you won't be alone. However it baffles me how come such a patchy platform became so big.
So, if you can use Wordpress + Woocomerce as it has been said here, go for it.
Best of lucks!
Go with OpenCart. Woo Commerce is a good choice but It is not organized like OpenCart. I used OpenCart to run an eCommerce website, and the experience was great and easy.
I do not recommend Woo Commerce or expression engine because I do not like using something is not built for its main purpose.
Just check the marketplace of OpenCart, and you will find something amazing.
PS: I've been using OpenCart since version 1.3, and I would still recommend it for 'Small Businesses'