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.

https://www.thelifeoverseas.com/xiaomi-3c-vs-3h-vs-pro/

My suggestion usually is to keep the selection within a span of 75% of the maximum range of the air purifier.

As such, Xiaomi 3C is 22 - 38m2 and Xiaomi 3H is 26 - 45m2

So at 75%, if your room area is smaller than 28.5m2, then the 3C would be suitable and if its bigger than the 3H is a better option.

Anyhow, for living area usually two small air purifier is better than one depending on layout.

The reason for 75% is so that, in event that there is abnormal pollution in the room, you know that your air purifier is still able to cater for it.

My opinion