In terms of an open source platform, Magento, OpenCart and PrestaShop are all fairly evenly matched in regard to security. osCommerce is probably the platform with the most active history of insecurity.
Look for providers that are PCI compliant. A lot of open source options will lack PCI compliance though. So, it may make sense to look at third party providers like Shopify, BigCommerce, or Magento Enterprise.
The developer in me tends to lean towards open source options that I can customize to my exact specifications. However, the realist, and the smart businessman, in me tell me not to try to re-invent the wheel and go with an established ecommerce provider.
In the end, it just comes down to what your needs are.