This really comes down to what your business question is. Do you want to know how many times the software was downloaded? Or do you want to know how many different people downloaded the software? Both are valid questions, but ultimately which one is the best approach will depend on your business.
For example, if every time the software is downloaded it uses resources of some kind, then you would want to know exactly how many times it's happening. If you're more interested in the reach, knowing how many different people (or devices) are using the software, then the unique visitors would be better.
It's going to come down what kind of question you/your stakeholders are trying to answer. You also might want to look at what other types of reports for similar things are being used by the business. If everyone talks in terms of unique visitors, it's best to match that to make it easy for stakeholders to understand.