Fresh air system and air purifiers are both used to filter pollutants, but the difference is that the fresh air system introduces the outside air to the room, while the purifier purifies the indoor air. The fresh air system has one advantage: it can bring in fresh air (fresh is equal to less carbon dioxide). But if the temperature difference between indoor and outdoor is large, introducing fresh air needs heating/cooling, and thus more power consumption.
What is the exact difference? The rated air volume of Xiaomi is 300m3/h, and that of the Blast is 1800m3/h. Moreover, Xiaomi claims that the ventilator is suitable for a room of 105 m2, but according to the national standard, a purifier of CADR=300m3/h can only match a 36 m2-room.
Since the ventilator is always filtering the dirty air outside, the lifespan tends to be shorter, and the primary filter needs to be cleaned several times a month. While the Blast is light, quiet and energy-saving, with longer filter lifespan (changing the HEPA filter biyearly and washing the pre-filter monthly approximately)
If you care about indoor carbon dioxide very much, you can open the window for ventilation when the air is of good quality, or open the purifier at the same time.
Learn more: Do Air Purifiers Work With the Window Open?