Which simulation tool you use depends on many things, including your application, task you wish to perform, your experience level with mechanical systems, which other physical domains your simulation will include (mechanical, electrical, ...), if your application includes a control system, and your budget for software purchases.
A big distinction between Simscape Multibody and any other multibody simulation tool is that Simscape Multibody is fully integrated with MATLAB and Simulink. If you intend to use your mechanical system model together with capabilities from MATLAB and Simulink, such as control design, optimization, hardware-in-the-loop testing, and post-processing, you will have a much more seamless workflow if you use Simscape Multibody. Direct integration of hydraulic and electrical system models is also easier with Simscape Multibody than other dedicated multibody simulation tools. Nearly any task in Simscape Multibody can be automated using MATLAB code and MATLAB Apps, which can speed up your work immensely and increase reuse of your models.
Cosimulation requires communication overhead between the two models/environments and places unnecessary restrictions on simulation step size. This makes cosimulation slower than simulation of the same model fidelity within an integrated environment. It is useful for validation tests where the physics or other behavior can only be modeled in a separate environment. It is usually more efficient to have a system level model in one environment to explore the design space, and use cosimulation on a small portion of the design space where you have to use it to get the answers you need.