The Coway 400G and Coway 250 both leverage True HEPA filtration and smart sensors to maintain air quality, but they cater to distinct needs based on room size and budget. The most significant real-world difference lies in their purification capacity. The 400G boasts a CADR Smoke rating of 416.0, enabling it to effectively clean larger spaces up to 650 sq ft with 5 air changes per hour. In contrast, the 250 offers a CADR Smoke of 248.0, suitable for rooms up to 387.5 sq ft. For users with large open-plan homes or those needing rapid purification in expansive areas, the 400G will noticeably clear pollutants faster and more thoroughly. This means fewer visible particles and odors, creating a healthier environment with less waiting. Its higher capacity also implies it can run on lower, quieter settings more often in moderately sized rooms while still maintaining excellent air quality. The 400G also includes an Energy Star certification, suggesting better long-term operational costs for its higher output. Price is a major differentiator, with the 400G at $749.0 and the 250 at $399.0. The 250 provides exceptional value for its price point, making it an ideal choice for bedrooms, home offices, or smaller living rooms where its 387.5 sq ft coverage is more than adequate. Both units feature washable pre-filters, reducing some recurring costs, but the 400G includes two sets, potentially extending the lifespan of the more expensive internal filters. The 400G further distinguishes itself with a minimum noise level of 22.0 dB, an impressive feat for an air purifier of its power. This makes it virtually imperceptible in a quiet room, a critical factor for light sleepers or those who prioritize a serene home environment. While the 250 is also generally quiet, the absence of a specified minimum dB means the 400G offers a guaranteed advantage in this regard. Furthermore, the 400G provides a more robust 5-year warranty compared to the 250's 3-year warranty. In practical use, both models feature smart sensors that automatically adjust fan speed based on detected air quality. However, the 400G's 'Eco Mode,' which can completely shut off the fan when the air is clean, offers an additional layer of energy saving and quiet operation, enhancing its appeal for premium users. The trade-off is clear: the 250 offers strong foundational air purification at an accessible price for average-sized rooms. The 400G, despite its higher cost, delivers superior performance, extended