[tjohns]

AirNow uses official EPA sensor sites, which are professionally maintained but fewer in number. These will have the highest quality data.

That said, the PurpleAir devices use high quality laser particulate sensors, so they're still significantly more accurate than most consumer devices.

Both are real time, but during a wildfire where smoke is shifting between neighborhoods, I'd go with the PurpleAir site to get a score for your particular neighborhood. Just be sure to turn on the LRAPA scoring model, as others have mentioned.