The big difference between the two for you, right this moment, is that WordPress.ORG allows for plug-in installation, and WordPress.COM does not. So, unfortunately, as a WordPress.COM user, you won’t be able to make use of the Sensei plug-in. But, if you decide to become a WordPress.ORG user, it will be available to you. Does this make sense?
If you are considering making the move from WordPress.COM to WordPress.ORG, check out this link: