Keystore
The Keystore is an encrypted Private key and can be very hard to transcribe, therefore, it can be stored on your own offline device, like a USB drive. Try not to store it online or on a server that uploads to the cloud. Make sure to store your password and Keystore separately because it is difficult to crack into the wallet with only the Keystore.
Mnemonic phrase
When storing your Mnemonic phrase, you need to be more cautious because the Mnemonic phrase is not safe at all if a third party gets control of the information. It is critical that you copy your Mnemonic phrase on paper, and keep it in a safe place. Make sure to double check the accuracy of your transcription and also make sure to think about how to store your Mnemonic phrase long-term.