Which should I use as the main mesh point to connect to the modem?

For MeshForce M1:

In a 2-Pack or 3-Pack MeshForce M1, any mesh point can be used as a main mesh point. There is no difference.

For MeshForce M3 and M3 Dot:

MeshForce M3 is the main mesh point (the white cube)
M3 Dot (the wall-plug WiFi) cannot be used as a main mesh point.