The +18/+24 dBu switch on the back of Matrix determines the maximum level of the A/D/A converters.
Neither setting is better than the other, you should choose the setting that best matches the operating level of the device you are connecting.
The following can also be considered (e.g., if the operating signal of the other device is not known or falls in between):
On the input side (D/A converter):
0 dBFS (digital maximum) = +18 dBu analogue --> More headroom, less chance of distortion in the Matrix.
0 dBFS (digital maximum) = +24 dBu analogue --> More signal.
On the output side (A/D converter):
+18 dBu analogue = 0 dBFS --> Higher signal level, more risk of digital clipping in the receiving device.
+24 dBu analogue = 0 dBFS --> Lower output signal, less risk of clipping in the receiving device but weaker signal.