I have been using both Magento (community edition) and Ubercart depending on client's target hosting preference. I must say that Magento has some impressive functionality and flexible as it has been around for a while. However there are some serious problems with this. Upgrading Magento to the latest version is a complete nightmare and have a good chance of needing to reinstall. Building your themes in Magento is so convoluted, it takes so many inheriting files to create a simple layout change. There is cummunity support, but most of them screaming for help as I remember. Magento model is "make money from support". So I found that they are not so quick to help on the forums.
With Ubercart it is refreshing to see the amount of active help. I think this is important if you are new this. If you are already familiar with Drupal, then I would say stick with Ubercart. It is much simpler to manage than Magento.
They are both good with application security, but you will need to setup your own SSL cert anyway. I've played with some other carts such as Zen and OS Cart, but found them inferior compared to Ubercart and Magento.