Is the decision whether to use PLECS Blockset or PLECS Standalone entirely based on whether I have MATLAB and Simulink?
Not necessarily. The choice is more a matter of preference. However if you do not have MATLAB and Simulink, PLECS Blockset cannot be used. While PLECS Standalone does not require MATLAB and Simulink, its optimized solvers provide significantly faster simulation speed. PLECS Blockset provides access to the MATLAB command line interface and workspace, as well as the Simulink libraries and toolboxes. The major functionalities inside the two versions are essentially the same.
Criteria such as whether you prefer not to work with MATLAB and Simulink or have limited licenses available, if simulation speed is critical, or the total cost of your software tools may all factor into this decision.