Deeds that are prescribed and deeds that are encouraged for the sake of Allah (SWT) are clearly distinct in Islam. Zakat is obligatory upon every Muslim. Whereas Sadaqah is an encouraged, voluntary deed. Most importantly, both earn the pleasure and reward of Allah (SWT), despite the difference between them.
Islam considers both an act of charity. However, performing Zakat is one of the five pillars of Islam. It is required of every Muslim to fulfil in each year of their life. This is as long as they have the capacity and meet the criterion.
Both share many differences as well as many commonalities. Essentially, each make a huge difference to the wellbeing of humanity.