I believe that the Sensirion SGP41 is the slightly better VOC sensor and it also gives an NOx signal that can be useful:
"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].
That's been my experience with the BME688 too. Bosch likes to pitch this as being able to detect many various things, then leaves it up to the customer to experiment with heater profiles and machine learning to reach said goal.
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.
We were getting headaches but didn't know why. Turns out the building was recently remodeled, and the VOC sensor helped us root cause the air quality (we were opening windows to get rid of the flooring smell, but then inhaling toxic roof off-gases instead).
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.