How do I choose a backup method?
1. Private keys: I create a private key for each address using "Wallet> Private keys> Export". Using these private keys I can transfer my address to another wallet.
You would probably omit private keys along the way. This does not ensure that you also backup your future private keys. You can export your xpriv (master private key) and that will allow you to generate all the addresses.1. Private keys: I create a private key for each address using "Wallet> Private keys> Export". Using these private keys I can transfer my address to another wallet.
when you make a wallet with electrum the normal way, there is no way of omiting a private key. you can only freeze addresses. (normal way is making a new wallet with the random seed electrum generates for you)
and imported private keys are a different story though, the only way to back them up is to store them separately or store the wallet.dat file because seed doesn't recover these.
and you do not have access to "xpriv" (master private key) unless you open your unencrypted wallet.dat file and look for "master_private_keys" which is an unnecessary complication the seed is doing the same job and is easier to back up by writing it down.
You don't have to. Go to Console, type in getmasterprivate() and bam. You get your master private. As I mentioned, you should be backing up your seed and not doing these things. This is if you believe you can't use Electrum anymore in the future and you want to have your current addresses. 
