It is NOT true that the higher grade the filter, the better the purifier. The problem with this logic is that the higher-grade filter comes at a cost. That cost is air flow. Higher-grade HEPA filters will capture more particles, but they’ll let less air through. Thus, there’s probably a sweet spot in the tradeoff between air flow and particle capture. So the best HEPA filter is the one allow the most air through the fan in a designated period of time.