The AM2302 (or DHT22) supports power supply between 3.3V and 5V DC.
I have seen both possibilities in various tutorials, although it seems most people go with 3.3V.
Is it preferable to supply it with 3.3V, or 5V from the Raspberry Pi?
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You can power the device with either 3.3V or 5V.
People use 5V when the device is at the end of a long wire (many feet). The voltage drop along a long wire can make 3.3V operation unreliable.
Make sure that regardless of how you power the device the pull-up on the data line is to 3.3V. The Pi's GPIO should not be exposed to more than 3.3V.