When should I use IP Address restrictions vs HTTP Referer restrictions?

IP Address - API Keys used server-side to make REST API web service requests should be restricted by IP Address.  The IP is the external IP address of the host.  

HTTP Referer - API Keys used client-side with the JavaScript API in the browser should be restricted by HTTP Referer.

References

How should I restrict my API Key(s) to prevent unauthorized use?

API Key HTTP Referer restriction nuances