Both are part of the DJI Avinox Gen 2 system. The difference lies in torque, power, and weight:
M2 (Standard): max. 125 Nm, 1100 W, 2.65 kg
M2S (Upgrade): max. 150 Nm, 1300 W, 2.59 kg – noticeably more thrust, especially uphill and in rough terrain. Despite the higher power, it is slightly lighter at 2.59 kg than the M2.
The M2S can be configured as an upgrade when ordering (+249 €).