I just bought the Anker power core 20100+ and it came with 2 cables.One Usb C to Usb C cable, and One Usb C to USB A.
I am just wondering which one would give me better charge speeds for my Nexus 5x or if it wouldn't matter at all.
Your device comes with USB Type C which is equipped to handle higher power for charging and hence that is recommended over using the USB C to A conversion
....Rather than using four-pins as in the classic type-A and type-B, the type-C connector has 24 pins and is reversible, meaning it can be plugged in either way. It supports 900mA and, on command, delivers 1.5A and 3.0A over a 5V power bus while streaming data. This results in 7.5 and 15 watt power consumption respectively, as opposed to 2.5W using the original USB (current times voltage = wattage)
Also see a decent comparison of Type C Charging