When deciding which HTML preprocessor to use, you need to consider a few factors such as features and functionality, syntax and style, ecosystem and community, and performance and compatibility. Features and functionality refer to the kinds of features you need or want from the preprocessor, as well as how much control and customization you need over your code. Syntax and style refer to how you prefer to write your code; do you prefer a concise, minimal, or expressive syntax? Ecosystem and community refer to how popular and well-supported the preprocessor is, as well as how easy it is to find documentation, tutorials, or examples. Performance and compatibility refer to how fast and reliable the preprocessor is, as well as how compatible it is with other tools, technologies, web standards, and browsers.