Do you know the difference between Apache and Nginx?
If you don't know the difference between Nginx and Apache, then you should go for our default Nginx stack, as it is a faster webserver (some would say marginally so in certain scenarios, but it is still faster).
Do you want/need .htaccess rules?
If you need .htaccess rules (which do not exist in Nginx land) then you should go for Apache. Although anything you can do with .htaccess files can be done in Nginx - maybe just not as easily copy-pasteable from around the web, as Apache tends to be the more widely used webserver.
Nginx works just fine out of the box in our configuration without bothering with .htaccess rewrite rules for a default Wordpress or Concrete5 or similar CMS installation.
If you have no clue what the hell all this means - try Nginx, and if that doesn't work for you for some reason, heck go for Apache.
Are you installing a brand new up-to-date system, or something old?
If you are installing a fresh installation of something, whether it's Wordpress, Concrete, Laravel, Drupal or whatever - our PHP7 stacks should be your choice.
If you are migrating an old site, it is often required that you choose our PHP5.6 stacks. However, we recommend that you start out with a PHP7 stack and then update your core system and see if things work out for you. If all else fails, you can then try our legacy PHP 5.6 stacks.