Both Digimat and MultiMech are multiscale codes to enhance standard FEA through multiscaling. The main difference is Digimat is based on analytical formula for the microstructural response (Mori-Tanaka), while MultiMech uses FE microstructures.
I use MultiMech because of their microstructural damage mechanisms, and microstructural design flexibility. It tends to be more computationally demanding, but yields more accurate and detailed results.
Digimat uses an analytical model (Mori-Tanaka) to predict the local composite behavior. Mori-Tanaka is a good micromechanical model for 2 phase particle analysis. But if you want to consider geometry of particles, spatial distributions of particles and take into account the interactions of particles with a medium, Mori-tanaka would not be accurate. The simulation will take less time as Individual mechanisms of damage and failure are blended into a single criteria but the results will be less reliable outside the assumptions of the model.
Digimat’s setup requires some coupon level testing to calibrate the model, making the modelling semi-empirical. Apart from having the capacity to handle semi-empirical modelling, MultiMech requires deeper knowledge of the individual components of the composites, basically giving you the capacity to model the constituents directly.
MultiMech is more powerful tool if you are interested in detailed analysis. As the complexity of your constitutive components go up, reliability on analytical micromechanical model would go down. Finite element analysis is advantageous in such scenarios.
I am currently using MultiMech for analysing nanocomposites (in fiber reinforced composites) and so far it is working out to be good. And apart from that people at MultiMech are really friendly and always ready to help!