The advantage of the annual subscription is that it entitles you to free SDK version updates as long as you have an active subscription. Also, it allows you to host (https://www.wikitude.com/external/doc/documentation/studio/studioeditor.html#hosting) AR experiences in Wikitude Studio and link them in your app, including 100GB storage space.  
On the other hand, the one-time fee does not include SDK version upgrades, no hosting feature and only 50 GB storage in Wikitude Studio.