I do not own an Air Quality Egg or an Aranet so I cannot really comment on both. In general most of the low cost monitors use similar sensor modules, e.g. Plan

I do not own an Air Quality Egg or an Aranet so I cannot really comment on both. In general most of the low cost monitors use similar sensor modules, e.g. Plantower for PM, Sensirion for TVOCs etc. What is important to ensure that the CO2 sensor is an NDIR or photo accoustic and not using estimated CO2 via a TVOC sensor that is very inaccurate. I would start measuring PM, CO2 and TVOCs in the house to first understand more about how well the HVAC system works. Then optimize it for low CO2 and PM. This should then flush out most of the internal pollution sources as well.