There isn't much difference - Square Enix did a REALLY good job of making both versions of the game essentially the same. You can use a USB mouse and keyboard with the PS4 version (even ones with remapping support, as long as they use onboard memory) and controllers with the PC version.

If you just want to play on PC hardware but don't want to ditch the controller, then keep in mind that even though Wal-mart and Target want you to believe that wired PS4 controllers don't exist, they actually do. If you want to avoid the headaches that wireless controllers sometimes cause people on PC, get a USB PS4 controller.

If you want to use a mouse and keyboard, however, I would recommend that you get a serious MMO mouse. It is by no means a requirement, but having to switch back and forth between WASD and ability keys can be frustrating for many people (like myself).