Use the browser Developer tool to inspect the decision radio button of the task form, where you will get the Value of the button. The button values will be integer numbers and not the text like Approve/Reject when used on SharePoint On Premise.

Use Confirmation message section to display the message when the user responds to the task.

If this does not answer your question, please do share screenshot of the problem for more clarity. 

Hi Chris,

As far as I know you are only able redirect based on the form being submitted / saved successfully and if something goes wrong you can send a cancellation form.  This does not refer to the decision that was made.

I also know you can store the decision in a variable as part of a flexi-task.  If you use the redirect function together with querystring variables or even create redirect pages with the same names as the decision and make that part of the URL redirect, won't you be able to do a dynamic redirect then?

Article about Query String Variables - https://community.nintex.com/community/build-your-own/blog/2016/08/01/pre-populating-your-form-controls-with-query-string

I'll read about it more

Kind Regards,