Both the platforms are different from each other and honestly, both have their pros and cons. Where Mac laptops are smoother to use, Windows provides a user-friendly interface and has wide range of supported apps.
That being said, such vague comparison among the two computer giants is next to impossible.
However many times it is seen that Windows PCs are a bit cheaper as compared to Mac. If your budget is limited, I would recommend you to for a Windows-based laptop.
About buying a new or used laptop, no matter which laptop you choose, I would recommend you to go for a new one, especially when you intend to use is extensively. You can go for a used one but only when the source is trusted.