Hi Joost,
Both can give stresses so that is not a differentiator. A few things will determine the suitability:
1.) Complexity of belt: If there are specific polymer properties, Mechanical might be better (more options)
2.) If there is a long simulation duration, Motion can be better. (Mechanical typically has more convergence issues).
3.) If there are millions of nodes in the model, Mechanical is better (SMP,DMP scaling is better in Mechanical)
4.) If roller placement is dynamic, Motion will give the option to automatically place the belt in the right location, so CAD creation for the belt is not required. Caveat for Motion Belt - Belt is not treated as continuous objects in Motion. It is created as a multi-linear object where the belt is connected using small slices/sections. A little bit of tuning is required to arrive at the right properties that reflect bending and tension well.
Thanks,
Ankush