I strongly recommend you get some formal instruction, such as the book "Pro Drupal Development" or "Front End Drupal" as well as get accounts at BuildAModule.com and/or Drupalize.me to get a deeper look into Drupal. The decision is ultimately one where you need to figure out your own level of technical competence - creating an eCommerce web site is complex, with legal ramifications if you screw up. So, this is serious.
Magneto is a good platform, but they just got bought by eBay, and in my mind that places their structure (support, UI, community) in an uncertain position. Drupal's not going anywhere but up.
I suggest you create one or two non-eCommerce Drupal sites first, just to learn the environment. Do this on your local machine, so you get personal experience with where files go, their organization, how to add and remove AND update modules. How to configure a what-you-see-is-what-you-get editor, and so on. These are all essential Drupal skills that take time to develop; not a huge amount of time, but time none-the-less that you NEED to spend before you take on the larger and more serious task of creating your online store. After that, move one of your web sites to an online host; forcing you to learn how to manage and maintain a remotely hosted Drupal site. Another essential skill you do not want to be developing with live customers.
Frankly, I would never recommend creating an online store as your first exposure to any eCommenrce environment. You need to be familiar with the CMS first, to insure you are not doing basic entry-level mistakes; such mistakes in a live online store could bankrupt you, land you in jail, or even expose your customers to massive credit card fraud.