Well, actually, the private key on the paper is probably not encrypted whereas the private key on the very nice piece of wood is encrypted by design - you would never want to give the person carving the wood your private key, right? So, they only accept encrypted private keys.
The private keys on each paper wallet I create are encrypted (: I use this, https://github.com/pointbiz/bitaddress.org. It has the built in area where you can input a encryption passphrase. Which is also the same code I used to create my encrypted private keys for the wood wallet
SO, u need to remember a password to decrypt the wooden wallet ...is not it ?
There is no a single answer, paper is fine too. Aluminium also is beautiful. 
