On a single-user system (Windows 7), does it matter whether you set PATH variables in User-variables or System-variables?
Why choose one over the other?
It depends on the use case. As stated by @Oliver Salzburg System-Variables affect every service for every user. For example it's needed when running planned task in the context of another user.
If you are just trying to run batch scripts as your user the User-Variables are a great place. Other users won't get affected by this.
You should be careful when changing this variables completely because some things on the system can break because the path to some necessary binarys cannot be found anymore.