Ethernet and WiFi

I have a Wifi channel on my pc, and also on my router.
I have a network printer attached to the router.
Currently my pc is connected to router through ethernet wired connection.

When I need to access the PC from a laptop by Wifi, is it best to use the PC WiFi, or the router Wifi connection, and how do I stop the PC connecting to the router by Wifi if that is enabled on both devices? I presume the ethernet connection is faster, and certainly more reliable.

Your laptop should connect to the router, the laptop won't even see the desktop's wifi to connect to unless you have configured it to be a hotspot (leave the PC's wifi disabled). Provided both the PC and the laptop are running Win 7 or later OS, you should be able to create a Homegroup so that you can share files between the 2 computers.