Simulink works numerically, you can not solve differential equations symbolically in Simulink. If you want to obtain symbolic solutions, you have to use MATLAB environment. If you want to obtain numerical solutions, you can both use MATLAB and Simulink environment. My advice for you is to use Simulink for numerical solutions as well because you can put Scopes to each point of your differential equations and plot the results easily. In MATLAB, you need to know the syntax and some manipulations to do so. To sum up:
-Symbolic approach requires MATLAB environment.
-Numerical approach can be done in both, my advice is usage of Simulink for that.