Gaia uses 3 redundant dust sensors, while Purple Air has only 2: The reason for using 3 redundant sensors is that, when one sensors fails, it is possible to know which one. In the case of purple air, with only 2 sensor, it is not possible to know which one fails, thus making the Purple Air data useless.

Gaia has more data than Purple air: For each sample interval (1 minute), you get the minimum, maximum, standard deviation, average and median for for interval. Purple Air only provides the average value of 2 minutes. With this additional data, you can get much more reliable information about the sensor data quality.

Gaia last longer. The GAIA station uses 25% duty cycle for dust sensors (active 90 seconds every 6 minutes), while Purple Air is using a 50% duty cycle. This means that the reading accuracy from the Purple Air quality decrease at least twice faster over time.

Gaia has a better mechanical design, with a vent cap, which protects the sensors from direct wind and rain. The purple air does not have any cap, so the wind can bias the readings on the Purple Air.

Gaia station cost $200, while the basic Purple Air costs $229. And for $200, you get 2x5 meters water proof power cable and the power supply for free, while for purple air, you need to pay an extra 40$ (ref).

The GAIA station sensors can be replaced by anyone. While for purple air, only the high-end $300 version supports user replaceable sensors.

The GAIA station has an external omnidirectional antenna. The external antenna performance is much better than the internal directionally antenna used by purple air. And using an external antenna, it is possible to use high gain antenna.