Which one would be better for macOS?

Hi,

I have a ThinkPad X1 Carbon 2nd Gen and I plan on installing macOS on it. This laptop is going to stay in one place so I can live with an SSD dangling of the side.

It has a 256GB M.2 Samsung SSD and I also have an external 512GB WD Blue SATA SSD.

I wanted to know which one would be better (performance and troubleshooting and also just peace of mind) I'm also going to be using Windows on the same machine. 1 OS on 1 drive.

Thanks :)