The difference between the two is that while guest self registration page is only meant to register the user by taking their details and creating an account for them, Hotspot
transaction allows the same users to be able to select a paid plan.
The power that using a hotspot enabled guest self registration page provides is that, you can now integrate payment gateways to the registraton form. The users can be made to pay money and accordingly be provided different internet browsing speed.
This enables you to have more control over the internet and badwidth usage allowed to every individual user.
You can have different bandwidth be allocated to different users based on the amount they paid.