I'm dealing with a Magento 18.104.22.168 Community Edition installation with various undocumented changes to the core (mostly integration with an ERP system), an outdated Sweet Tooth Points & Rewards module and some custom payment providers. It also doubles as a mediocre blogging/CMS system. It has one store each for 3 different languages, with about 40 product categories for a few hundred products.
The client is planning to redesign the site, and is considering upgrading to Magento 1.6 or 1.7 if it has improved from 1.4.
The client is also open to other e-commerce systems. I've looked at OpenCart and it seems to be quite developer friendly with a fairly simple structure. I found some complaints regarding its performance when the shop has thousands of categories or products, but this is not an issue with the current number of products my client has. It seems to be solid ground for easy customization to bring the rewards system and ERP integration over.
What should the client upgrade to in this case, are the newer versions of Magento genuinely better or should I consider another system entirely?