1. How the BIP38 exactly works? And how is it different from non BIP38 encrypted paper wallet?
2. Would you recommend more encrypted paper wallet or bitcoin-qt? If bitcoin-qt, will deleting it from pc help?
Thank you very much for your help
First get the private key from your bitcoin address. See:
http://bitcoin.stackexchange.com/questions/7536/how-do-i-export-my-private-keys-from-my-bitcoin-qt-clientYou can print this key out or write it down. I would advise against encryption. The biggest danger is that you forget the passphrase over the years. Has already happened to me. Is not a nice feeling. Do not underrate it. I am a professional, and even it happened to me.
For re-import see:
https://en.bitcoin.it/wiki/How_to_import_private_keysIf you need to encrypt printed private keys, do not overdo it and take only passwords that are buried deep in your mind.
I'd probably would just use my GPG key. I have stored my GPG key at many places (in all my mail clients for example). Someone who takes the GPG encrypted paper wallet, will hardly even have my GPG key. And a hacker who steals my GPG key, does not have the paper wallet. Not a perfect protection, but good enough.
But I will have a look at BIP38. I do not know it yet. Thanks for the note.