I think 3C and 3H coverage area differs but filter is the same if not mistaken.

As for the auto mode, I use automation to increase the speed. Its more practical and useful this way; can also set it to off for few hours of the day (since I use it in bedroom, I keep it off between 8 - 3pm and my automation setup is such that once it the Air Quality is above 40; I set the speed to medium / high and then drop to low below 35.

My brother smokes in his bedroom next door, and secondary smoke comes into my room; around AQI 40; I can get the cigarette smell; hence, I set my automation this way.