HI, @TomCawthorn, I think both servers are good, but i prefer Nginx because of its more efficient architecture, the less verbose configuration it lets you do and the performance.
Nginx with php will work as a proxy, because Nginx can't process php code itself, so it will pass the request made to a php file to the php-fpm cgi gateway.
The same is for the new Apache.
If you use the old Apache way that let's apache process a php file request using an Apache "module", then there will be a performance problem with a 1 million users.
This is my opinion based on many time using apache and now joying using Nginx.
Also, i have learned a lot of this stuff from @fideloper and his great book ServersForHackers.
Hope this helps you!