I would implement a modal and not a true pop up. Only do so if there is not a cookie already telling us the user’s choice. By setting a cookie, you do need to address GDPR and other related privacy concerns. A cookie is the only practical way to keep track of a random user’s choice over an extended period of time.

Create a function that outputs the necessary form and JavaScript to collect the user selection and save it in a cookie. Add the necessary CSS that displays this output as a modal. JS then hides the content by changing the appropriate CSS properties after a selection is made.

Insert on the appropriate template code that conditionally calls this function if no cookie exists. If a cookie exists, get its saved value and redirect accordingly.