Is there a reason to add a variable to one over the other if I only ever use one user account?
In your case, in general, no, not really (since you are probably also the administrator of the system).
To modify a System variable, you must be an administrator on the system. All users can modify their own User environment variables. In general, System variables will need a restart to become effective, while User variables will become effective immediately. Note however that after a change to the User environment variables is made, any open software programs should be restarted to force them to read the new registry values.