Both air purifiers are cool to buy. Although, when considering which Dyson air purifier to buy, it's essential to be aware that there is some feature which is missing in the HP01 such as the Backward mode, Carbon filter and real-time Air quality reporting display. However, it's cheaper compared to the HP04.But if you think having the upgraded feature is essential, then it's better to consider buying the HP04 over the HP01. But be reminded that it's costlier than the HP01.