When looking for Drupal native e-commerce carts its limited to two packages Ubercart and Commerce. Ubercart is the long leading one and Commerce is the new one, but developed for drupal 7. Commerce is a bit complex when compared with Ubercart but it offers more features like shipping, inventory control etc. And another issue with Ubercart stores who want to upgrade to Drupal 7 is, lot of Ubercart contrib modules weren't being upgraded to 7. So the solution is Commerce.