Corentium Home by Airthings: 166
Safety Siren Pro: 238
The safety Siren Pro is supposed be calibrated annually by the company, but we haven't paid to have that done. I think I saw someone say it's $75 + shipping? I like that Corentium Home's calibration is supposed to be good for 10 years.
I haven't been a big fan of the Safety Siren. It only has 7-day and long term readings. I like Corentium's 1-day reading - I can tell more quickly if a change in the house "seems" to be having an impact. There are so many factors, it can be hard to tell, but waiting a full week was a long time.
As well, Safety Siren sounds an alarm whenever it goes above 200. Once the level hits 200, the alarm can be turned off. Great. Unfortunately, as soon as the level drops below 200, the alarm setting is turned back on. We often hover around the 200 mark, going just above and just below 200. Since I can't permanently disable the alarm, the alarm would be going off multiple times in a day despite me turning the alarm off each time. I have to remember to unplug the unit at night whenever it was around 200.