Which air quality sensor should i choose?

Hi, as a part of my project, I am working on an air quality monitor and a flammable gas detector. I have done a bit of research before selecting the sensors. After going through a lot of tutorials and articles, I used this Types of gas sensors article as a reference and thought of using the MQ series sensors for the project. The main positives I see in them are that they are cheap and will fit in my budget, and there are a ton of tutorials for these sensors. But, my question is, are they reliable? Do I go with them or should I choose some other sensor? Is the preheating period of 24 hours necessary? It would be really helpful if someone could point me in the right direction.