Depends on your app and app stage.
Paths are harder but superior for SEO & native GA tracking without as much need for custom events, also better if you’re using 3rd party redirects for any reason as many clear Params on redirect.
Params are easier and faster to implement but don’t have some of the benefits as paths.
For signup/login these should always be separate pages, regardless of what anyone says with using states to display the login/signup group, they are wrong. The only exception to them being separate pages is native apps. Your marketing team will thank you and tracking data will always be consistent and clean.