Is there a best practice for which one to use?
Many tutorials online take various approaches - the older ones seems to use mnemonic whereas the newer ones tends to utilize private key.
I understand that the mnemonic is the parent of all it's private keys - so I can see logically how the usage of a private key would be better in this sense.
Truffle seems to suggest
I suppose this is related to the usage of
HD Wallet Provider
Hardhat seems to suggest
In the case of Private Key/ Mnemonic:
 Is there a known best practice in this case and where is it defined?
 What is your preference and why have you chosen it to be so?