Simulink provides a graphical editor, customizable block libraries, and solvers for modeling and simulating dynamic systems.
It is integrated with MATLAB®, enabling you to incorporate MATLAB algorithms into models and export simulation results to MATLAB for further analysis.
The Simulink approach is based on time based and multi rate system.
SO that will be useful for HDL code generation.
Whereas, MATLAB is for the mathematical based algorithm development and which will not consider the time while in simulation (independent of time).
Simulink is graphical and more interactive to the user.