It is totally your choice.
The GET Button Link gives you the GPL Version of Themes and Plugins. Developers use the GPL Version to evaluate the Theme/Plugins or for budget concerns. GPL version works the same as the original licensed version except for the updates and support.
At the BUY Button Link, you will to redirected to the Theme Forest Website to buy the licensed copy of the product by paying the full cost. You shall be getting the support and updates as in terms given by the Theme Forst.