Dear Reema, I've read your post and can empathize with both your staff and you! Because, in your own positions, you're both right. You want to know the 'tool' to evaluate their performance. That is your prerogative. They want to know 'how' their performance will be evaluated. That is their prerogative.
Answer to Your Question
You've jumped the gun by a few steps. A PMS tool will only help you once you've entered data into it. The more complex and analytical the tool, the more detailed its output. But you know that.
What you need before a PMS tool are Performance Criteria, Competency Checklists, Competency - Positions Mapping, Individuals - Competency Mapping, etc. Do you get the point? It's very tempting to want to manage people's performance. However, the problem arises when those same people demand to know the criteria, its degree of fairness, clarity, rules of engagement, appeals process, etc. And no PMS tool will answer these questions.
So, if you really want your PMS to be thoroughly effective, fair, transparent, and accepted by all, please speak to some professional HR consultants and get them to help you. It will cost you some time and money. But believe me, only then will any PMS be of service to you and worth your efforts.
And the biggest advantage you have is that you only have a workforce of 15 to work with.
I hope my answer helps.
Kind regards,