Who cares what the admin interface looks like, that’s so easy to change… As for Magento, it seems actually pretty great, it just requires a powerful dedicated server to run smoothly which makes it only suitable for bigger shops.

Opencart has a long way to go, but I think it might be a pretty good base if you wanted to start a new branch. The one thing these carts really lack is some sort of hooks system that makes plugins really easy to integrate like Drupal provides.

The issue I was bringing up is not how the cart functions as an ecommerce tool so much as the ease in which one can customize it. Yes, each of these carts can be customized no doubt, but some are far easier to customize than others.

My ideal cart will have very little inherent functionality outside of the basics of a cart, and the code base should be clear and well organized so I can easily change things that need changing and add things that need adding.

After looking over Open Cart’s code again, it may be a good base for me to use. I do remember being disappointed with it’s interface though and I felt the administration panel was lacking.

The only thing perhaps that makes a difference among the ones you listed is whether or not they follow the principles of commerce. Meaning no matter the OOP that sits behind, the objective is to make sales.