It appears the Purple Air uses the Bosch BME688 while the AirGradient uses the Sensirion SGP41.
I’ve used the Kaiterra Laser Egg for detecting VOCs previously, as well as some off-brand handheld device off Amazon. They mostly agree.
"The testing results show that the SGP41 has a better response for 9/10 tested VOC solvents and can detect the level of NOx gases, which is a unique and important feature for hazardous and toxic gas detection and alarm systems."[1]
In general measuring VOCs is quite challenging and people really need to understand these type of sensors. A couple of months ago, I wrote a whole blog post about these issues. [2].
SGP41 algorithm is open source, BME is closed source, requires you to link a binary during compilation.
In theory BME can sample additional gas profiles (programmable heater waveform), in practice it's useless.
I agree that the SGP41 has better sensitivity, and also faster response and better stability.
The results I've seen for methane is discouraging, to say the least. Now moving to a NDIR sensor. A completely different thing, I know. Guess I was under a spell, it sounded too good to be true - a BOM cost reduction of 20x, and it was. I can go on and on about this one and the driverlib for it, but I've spent enough energy already.