For that bed size, you’re at the border of where the extra resolution of the 8mp is going to make a difference. Depends on how much accuracy you want
The 8mp N (Narrow) will have less wasted space around the edges than the 8mp W (Wide).
Keep in mind that the Narrow does not work on Mac OS, for reasons we have not been able to track down. If you’re on Windows, great! If you’re on Mac, you’re going to need the Wide version.