Need help optimizing a site for speed and response time
You can use both as per my knowledge.

For Drupal generally, Varnish works for unauthenticated users traffic and memcached will cache authenticated users traffic.

Memcache saves your DB from doing a lot of read work, Varnish saves your dynamic web server from CPU load by making you generate pages less frequently.

But in your case I think Varnish is better option.