Geant4 is a widely tested and capable Monte Carlo Radiation Transport code
Most simulations can be created using gmad, mad(8/x) input syntax to build simulations. There is no need to know C++ typically needed when developing Geant4 applications
BDSIM is completely open source and new features can be added using flexbility of C++
BDSIM provides a set of generic accelerator components like bends, quadrupoles etc.
A BDSIM simulation is not just of the accelerator material but also important surrounding material like the tunnel or shielding.
Simulation output can be recorded in accelerator curvilinear coordingates, at planes (samplers) or quantities can be measured in scoring meshes.